Not a bad film

Mona is a mystery for me too. Not showing any specific reason why she gave up her better life may be a way to make the movie more powerful, I guess. A similar movie is "Five easy pieces", which also doesn't give any specific reasons why Jack Nichlson gave up his brilliant pianist career to work as an oil worker. There may be different reasons for those tramps(tramp may be a better word than vagabond for Mona), including some mental problems(like antisocial personality disorder?), but I think the pessimistic hatred to the current rules of society should be the main reason, they have delibarately choose to lead their lives by their beliefs, not by their mental situations. In seeking their independence and freedom, they say no to anyone who wants to help, just for showing their human dignity, I guess.

By the way, what is the title of your book? In French or in english?