Marius Was Kind Of A Jerk
In the movie, thirty seconds after Cosette and Marius have gotten engaged, Jean Valjean sings my favorite song and confesses all to his future son-in-law. Marius grasps the significance of the story, but tries to get Valjean to stay, finally conceding with "For the sake of Cosette, it must be so." Not so in the book. In the book, Valjean waits until after the wedding to tell Marius about this. Marius assumes the worst and decides that he's going to limit Cosette's interaction with her adopted father. This causes Valjean to lose the will to live.
Conclusion I hope that you have a more complete understanding of the story now. I've enjoyed going over some of the finer points of this great novel and hope that you may pick it up next time you're in the mood for a great read.