I don't get how Les Miserables is so miserable

It’s a play about a violent time, and how the people involved conduct themselves in the context of their society. Jean Valjean tries to lead his life in a moral fashion (forsaking theft, establishing a business, caring for others - even to the extent of risking his freedom). Fantine is doing what she can to provide for her daughter, Cosette. The rebelling students trying to bring about a more just society.

The book, and the play, are written more for social commentary than entertainment, and the society at that time was rather brutal. (Another period piece that is also a musical is Charles Dickens’ “Oliver Twist”.)

Most people (including me) know the story from the musical, and appreciate the music first, and then the story.