My ex boyfriend is a new person now, and it's scary.?

My ex boyfriend pledged a fraternity and dumped me for some other girl who was in a sorority. I felt awful at the time, he and I were together 3 years and were talking marriage, he got into a frat and that was the end of that? what makes people so superficial? I'm over it, but he isn't because he still calls, but I don't answer the phone b/c he treated me so bad... what makes people mean when they get things in life?