Wow, I'm really sorry that this happened to you. I'm just as clueless as you are as to why he would break up (I think he was telling the truth when he said you didn't do anything).
That's what important. If you didn't do anything, then the problem is him. Some guys freak out if they start to have feelings for someone else, so they end the relationship. (the whole "I'll end it before you can hurt me").
But if you don't think he was falling in love and refusing to admit it...Sometimes, people just need space. I'm not saying that makes it okay, but atleast you could understand why. People, especially teenagers, are very protective of their time and, more importantly, their heart. Suddenly sharing both of these things w/ another person takes some adjustment--and it's an adjustment some people aren't willing to make.
I don't know if this helped any, but I hope you atleast realize that it obviously isn't your fault. So, just in case you're blaming yourself, don't. It'll still hurt, but time (and also chocolate ) heals all wounds.