Why do some friends just ignore you like that??? it's unbelievable.?

I'm in my last year of high school. I have a friend who now ignores me because of someone else. We've been friends for about 8 years now. Just some months ago, a new person called Jade, started my school. My friend, dave, became good friends with her. Now all of a sudden he has completely forgotten me. He even pes me in corridors without even saying a word to me. He deleted my contact from his yahoo messenger. Now i just found out he had a birthday party and didn't even invite me? how can that be??? we tell each other everything. I'm really sad about it, and now i don't even know whether i want anymore friends, if that's what friendship is all about, i'd rather be alone. He recently sent me a three word email saying 'how's it going'. But i didn't reply. He pretends as if he doesn't know what he's done. I think i'm done with friends as this isn't the first time this has happened. Maybe i'm better off alone.