The greater good.
Montparnasse is in love with Éponine.
Everyone think he doesn't, that he just wants her ass, some sex and don't really care about her. Well everyone can go fuck themselves.
How many time he got hit by the old Thenardier when he try to protect her. How many time he propose her to stay to his house when she got afraid of his father. How many time he got heartbroken that she refused and went to Grantaire's instead.
And Éponine is not dumb. She knows he's in love with him. Unspoken words between them but she knows. That's why they stop sleeping together. She knew only by looking at him how it hurt him. And he fell in love harder with her because of that.
Because she clearly stated that she can't date people that have as a job to sell drugs. Her father started like that and both of them know what he becomes. And the hard part in it was that he understood. He knew she was right, but you don't choose who you fell for.
He's not even mad when he saw her falling in love with some school boy. Because when you are Éponine you can aim higher. She deserved more than the life with her parents and hanging with friend like him or Claquesou. He hates that he can't give her what she need but he's not mad.
He got mad the night she totally came heartbroken because her Marius asked another girl's hand. He got mad because that guys, that's schoolboy who is supposed to be smarter than him just miss the chance of his life with Éponine
He got mad when, for three week she asked him to be constantly high because her pain was too much. He hated giving her sleeping pills pretending it was something else so she fall asleep on his couch. He won't give her some strong shit to forget. That's how you get hooked to drugs. And he doesn't want Éponine to be junkie because of some pretentious jerk.
He hated having to call Grantaire to pick her up when she fell asleep on his couch because he knew his place wasn't safe and she will be better in his bed. He hated not being to help her and keep her safe.
And now that she's happy and in love with another school boy he know he lost his chance. The guy feel the same about her, he's smart, always a book in his hand, make her laugh, take her to dinner. But Montparnasse knew he lost his chance when he saw her looking at him. She was happy, she was in love and she was looking at him like no one else matters.
And he knew that he couldn't break them up just for his selfish need to have her. Because that's what you do sometime when you are in love, you suffer in silence and accept that the other can be happy with someone else.