In "Les Miserables," why does Marius leave home?

In Victor Hugo's classic, "Les Miserables," he writes in Volume II, 6,

One day there was brought to his [M. Gillennormand's] house in a basket, something like an oyster bastket, a big boy, new-born, crying like the deuce, and duly wrapped in swaddling clothes, which a servant girl turned away six months before attributed to him.

This baby is Marius, son of Colonel Georges Pontmercy of Napoleon's army, the son-in-law of Monsieur Luke Esprit Gillenormand, an old Royalist.  The grandfather, who considers his son-in-law a "blockhead" because of his political affiliations, demands that the baby live with him.  The Colonel agrees under duress, knowing that the child will have no inheritence if he does not comply.  After Marius becomes a young man, through chance he sits one day in a chair that a man tells him is his; the name on the chair is M. Mabeuf, church-warden.  M. Mabeuf recounts to Marius how a certain officer would hide in the bushes to see his son coming from Mass on Sunday with his aunt since this was the only time that he could see him.  Marius, who has always thought that his father did not care for him as the grandfather had kept any communication from Marius, sets out to learn of his recently deceased father.  He goes to a law library and learns all he can of the New Republic, learning a new respect for his father.  When he returns wearing a black ribbon, the grandfather expects to find a portrait of a young lady hanging from it.  Instead, there is a portrait of Marius's father and a box with cards printed for Marius with his real name.  Enraged, the grandfather engages in a political argument with Marius and turns him out of his home.