What causes some nice guys to turn into jerks?

I broke up with my boyfriend a month ago. Everything was great until he suddenly went distant. After two weeks of not seeing him and barely hearing from him, he calls me and says he wants to go on a break due to his busy work schedule, but he hopes everything works out.

A couple days later when I was getting ready to go to a rave with friends, a friend of my ex texts me what sup. I asked how my ex was doing and he said that he was having fun with them. I felt suspicious. Especially when just a week ago, i asked if he wanted to go to the rave and he said yes.

i decided to confront him and he got angry and said he doesn't want to talk. I asked him if he wanted our relationship to work out. He said yes, and then I told him I needed time to think and that I'd get back to him.

Later on in the night, my ex texts me that he's sorry he's a **** up, that I'm the best thing that's ever happened to him and that he doesn't want to ruin it. I then later desperately calls me wondering if he got the text and said how stupid he feels. He also asked if i called him the next day.

The next day, i called him, and several times because he didn't answer. He then texts me "why i'm blowing his phone the **** up" and that he doesn't want to talk. An hour later, he calls to apologize for the way he acted. We met up later that day and I told him how concerned I was that here was something he wasn't telling me. He didn't give me any straight answers and said that there are things going on in his life that he feels like he can't talk to me. We got into a fight and he left me alone crying. That was the last time I ever saw him.

I later found out from a friend of a friend of his that he has been hooking up with multiple girls the entire time. I broke up with him and he told me to **** off and that i'm a psycho. Just a couple days ago I called him once and he didn't answer. i still have some questions and need closure. A day later, his friend calls me wondering how I've been. What makes nice guys turn into jerks? Did my ex ever care about me? Why would his friend try to talk to me?