Do people change?

I just had an interesting conversation with my roommate. It all stemmed from a chat about Sandra Bullock and her (ex) husband. My roommate said that she shouldn't really have been surprised that he cheated on her - in the end she married a bad boy. But I don't believe in that - I believe that anyone can change, and that it doesn't matter if someone cheated on someone else in the past, it doesn't mean that they are going to cheat on YOU.
Does that make me naive, or too trusting in people? Am I just going to be disappointed for the rest of my life because I believe in everyone until they prove me wrong? Because if this is true then I don't see the point in trying anymore, and if there is no point in trying, what is the point in living?

Honestly... I would rather go through life believing in people, and trusting that people CAN change, rather than never believing in anyone. That's one thing that I will never be able to change about myself.