What went wrong? Long story...

Okay. Granted this happened a few years back… but the wounds left were deep and the scars remain. I was in high school and there was this boy who lived in my neighborhood that I’d known all my life, but didn’t get really close to until my sophomore (his freshman) year. We became fast and good friends and since we lived in the same neighborhood, we would walk to and from the school on a daily basis and he would stop by with his dog on the weekends often. So when he asked me out, I was a little hesitant because I had recently broken up with my first, serious boyfriend and I also didn’t want to ruin our friendship. But after some consideration, I agreed, and so we began dating. Things were marvelous at first. I truly loved him and (thought) he truly loved me. We had trust, we had friendship, we had everything. It seemed like the perfect relationship. I knew that whatever I needed, all I had to do was ask him and he would do it for me. But after several months of dating (preceded by years of friendship), things started to go sour. Instead of walking to school with me, he got rides with a friend. Instead of meeting me after school to walk home, he avoided the hallway where my locker was located. I would call, he wouldn’t answer, I’d leave a message, he’d never call back. I would try to talk to him at school and he would avoid me. I was devastated. Our relationship was as serious as one could get. He wasn’t the first person I’d had sex with, but I was his first. The day I KNEW it was over was when I wrote him a letter that basically said: “If you care about me, you will meet me after school so we can get through whatever hitch this is in our relationship” and he never showed up. Sure enough, 2 weeks (less) later, he was dating someone else. His relationship with this new girlfriend lasted all of 5 or 6 days before he did something similar to her (even worse, I’ve heard). So my question is (nearly 9 years later) what went wrong? Why did he hurt me so bad if I didn’t wrong him? Why did he just start dating someone new without informing me of our break-up officially? I really have never understood and it haunts me to this day. Advice is appreciated. Thanks for listening to my story.