I think the real question here is what would hold water for you? There are hundreds, thousands of people in history who have done things that were not on the radar for other people of their social group. What pushes anyone? Bear with me as this will get involved.
"It's Russia. A landowner's estate is reckoned not in acres but in adult male serfs, and the agent of reform is not the rebellious slave but the repentant master. What a country! Napoleon had to drag us into Europe before our embarrassment at ourselves put seditious thoughts of reform into the heads of the returning army officers. I was thirteen at the time of the December revolt. One day, not long after the Tsar celebrated his coronation by hanging the Decembrists, my father took my and Ogarev for a drive into the country. Until I met Nick I thought I was the only boy like me in the whole of Russia. At Luzhniki we crossed the river. Nick and I ran on ahead, up to the Sparrow HIlls. At the top you could see all the roofs and cupolas of the city shining in the setting sun . . . and we suddenly embraced and made a sacred vow to dedicate our lives - yes, sacrifice them if need be - to avenge the Decembrists. It was the hinge of my life."
This is Tom Stoppard's words for Alexander Herzen at 22 explaining his radical involvement to a fellow radical. I use Stoppard because it's quick and punchy and when I heard these lines spoken on stage, my jaw dropped because I had never known Herzen before yet this was the very background, in general, I had already assigned to Enjolras and Combeferre. What Stoppard leaves out, however, is incredibly interesting: Herzen's father was not married to his mother. The surname Herzen is invented. The father was called Ivan Yakovlev, he was a landowner who brought his German mistress back to Russia with him but never married her. But as Yakovlev never married anyone else, no one actually cared that his family were technically illegitimate. Herzen was well-educated, his father was obviously in favour of certain aspects of the West, his mother was German, and so he grew up reading the romantic literature of the day and got in trouble for reading Rousseau's Confessions when he was 16. He went off to university at 17 and increased his radical activity to the point that he was arrested, imprisoned for nine months, and banished to the Urals when he was 22. Yes, radical activity in Russia was discussing socialism and emancipation, and sometimes Hegelian dialectics, but it's still deliberate action to prepare yourself to go against the government in even larger ways. He left Russia in 1847 and was never permitted to return, spending the rest of his life writing and arguing for socialism and a Russian revolution.
Beside him, his friend Mikhail Bakunin. Excellent family, wealthy, Bakunin was at the artillery school but started making a fuss over Poland more to make a fuss than anything else. His father tried to get him to either stick with the military or join the civil service, but he went to Moscow to study German Romantic philosophy instead. He became obsessed with Hegel, made it to Berlin (living off loans from friends, since his father wasn't going to pay for this rubbish), and only there discovered socialist thought. He jumped from thinker to thinker, finally alighting on socialism, which got him a demand to return home. He didn't return voluntarily, had his property confiscated (his father had lately died), and started bouncing around the German-speaking states instead, until he happened to end up in Dresden in 1848 and helped out in their revolution, which got him arrested by the Austrians who then sent him back to Russia for real punishment.
So what is the cause? Hundreds of men of this generation had the money and travel permission to go to the West, but most did not return with the idea of creating a socialist Russia. Stoppard tries to explain a bit, it's part of the point, but what is the explanation for someone not only deciding at thirteen that they are going to avenge the Decembrists but holding to that idea, those ideals, for 45 years? Is it the way Herzen's mother stuck to her lover, leaving her home when she was 16 and following him to a foreign country, remaining by his side until his death? It can't be the way his father abandoned a military career because he preferred to sit at home and be cared for and indulge in some hypochondria. Bakunin bounced from one thinker to another, but the constant was always that his father was oppressing him (by expecting that he get a job that paid money or else taking over the management of the family estates) and he had to get out to the West where everything was awesome. His actual implementation of revolution involved a lot of really bad plans, of letters written in code with the key sent in the same envelope, of helping Wagner burn down the opera house in Dresden because Wagner was pissed off at the director. Marx broke with him because Bakunin was all enthusiasm and no sense. But was the cause that his father wanted a normal life for him and quit giving him money to throw away? His family adored him. Was it that he was thrown in to military life so young? It pushed him towards Schelling and Fichte and Hegel - at the time Herzen was in prison for talking socialism, Bakunin was trying to get to Moscow to hang out with his philosopher friends. There's only a two year age difference here - they're exact contemporaries, friends, on generally the same side of history. But Herzen set his path at thirteen and became known only as an intellectual; Bakunin didn't discover that he even wanted to undo everything in Russia, as opposed to his own family and anything that annoyed him personally until at least the age of 28, but he participated in multiple risings, escaped from Siberia, and was known as a participant as much as the thinker who created anarchist thought.
How does one decide the cause? Obviously both men are very different, were very different as children, but came from the same social milieu with all the same benefits, and, unlike other men from the same social class, ended up leftist thinkers who paid a legal price in their home country for daring to speak what they thought.
Herzen and Ogarev bear a resemblance to my Enjolras and Combeferre (Ogarev was life-long friends with Herzen, is known as a revolutionary poet, and was one of Herzen's collaborators until his death). This was completely accidental, as I had drawn the outlines of that relationship in the couple of years before I ever heard of the Russians. But it's obviously a thread connected to real life, that it is possible to come to something early, without direct parental support (Ogarev's father was conservative, appalled by Herzen's arrest, and when Ogarev was sent home under police supervision decided that the boy could be cured of politics by setting him up with women instead). The linguistic ability and availability of Western literature in the home certainly made it possible, but that's very different from being a direct cause.
I almost think youth is a requirement here, that these things be taken up early if a person have a fixed character, or that a man's character require some of the changeableness and fixation of youth (Bakunin was sometimes, in later years, referred to as "Big Liza", after one of Herzen's daughters who was a toddler at the time, because he seemed to have that same mentality of putting all his energy into five different things in five minutes and expecting everyone else to follow these jags with the same utter intensity). That something must be discovered young and never dropped. But a fixity of character, a purpose that lasts for 45 years despite huge setbacks, are things that may be innate rather than developed, or that come from family rather than external forces.
So I'm not sure a "why" is possible. (Despard can go into Irish examples: we could start with Michael Collins being born to a Fenian, but his father died when he was six, and had retired to the country to set aside politics and start a family in any case. But he had local influences, teachers, in addition to a family tradition that he would one day do great things for Ireland. But how many people end up never living up to deathbed wishes? How many men who are lucky enough at 60 to marry a 23 year old have really managed to select a woman who will raise his children in the exact mode he would have done? Collins had to have the interest in the first place to be susceptible to teachers outside his family, and it was those outside influences that were the ones he remembered.)
But that's all really about how one comes to a particular set of beliefs that are considered radical by the people with whom you grew up. There's also how one takes setbacks, situational changes, calls on one's time - how one sticks with something that brings only emotional pain in the hope that eventually, it will all come right.
And there's also how one characterises both the system and a particular position within it. I don't think the things Enjolras is fighting for are going to affect his real material position - he's not a full-on socialist. As with Herzen, if reform is from the top down, Enjolras is the repentant master, and I have no doubt that if he had lived long enough to have estates or enterprises under his control, he would have made them into models of reform (or probably had Combeferre make them into models of reform). Proof that it can be done, that it isn't an unattainable goal. So the risk Enjolras runs is to his immediate life - that the risks are imprisonment or death - but not to his position or even very much to his wealth depending on how that wealth comes to him. If he even bothers to think about it (I think here of Stoppard's characterisation of Bakunin as not realizing that his father's farm is an agricultural business - it's just so easy for rich young men to think that money grows on trees because they've been able to be sheltered from the day-to-day dealings of their fathers).
anyway, this is a v. long way of saying "this is why I think what I have works, if anything can be said to work". And that the question is incredibly complex, this web of family and teachers and what can and cannot be perceived about inequality, and how the family is placed within the social context, how the political situation is framed both by the family and by the larger local society. I've tried to bring a few real people in so that one can see how complicated it is in real life. (also because if you want influences, I think my Bahorel is turning out to have a few Bakunin-like qualities. Although at least my Bahorel understands that his father's farm is a business that one of the boys is going to have to run someday.) I see it as an early interest in political issues (possibly from overheard conversations) compounded by someone pointing out the results of said political issues, and the encouragement of an outside party that such interest is to be applauded and indulged. Combeferre and Enjolras essentially manage to continue egging each other along, in the way I've constructed this. But it's a consideration for every single character, not just Enjolras even if he is the leader. What makes them willing to go to jail, to forfeit their privileges, for the rights of man? It's the part of the character building that is so often either ignored or too heavy-handed in response to how it's usually ignored. But it's also necessary to know the political context and what is actually being fought over. And that's always been pretty fuzzy, too, so of course the motives are fuzzy. (and Enjolras the Marble Lover of Liberty sprung fully formed from Marianne's forehead doesn't help.)
What kind of literature and what kind of life is the same question. - Tom Stoppard