Seriously mixed signals Options

[xCutUpAngelxxx]

Okay, I'm really upset with a boy that I've kind of had my eye on.
It's a long story, but I feel that I have to tell it in order for everything to make sense. Basically, my best friend liked a boy for quite a while(a few months) and he was just interested in being friends with her. Then, she introduced me to him and we started talking and he and I ended up liking each other. My friend found out and she was extremely upset, because I guess everytime she wanted to hang out, he'd ask if I was there and she still really liked him, but he liked me.
Luckily, my friend's pretty mature and not a complete bitch and she said people can't control who they like and she said she wouldn't be happy if the boy and I dated or something, but she also wouldn't end our friendship.
I promised my friend that I wouldn't date the boy, but I couldn't garuntee (spell check) that absolutely nothing would happen between us.

Well, now, about four days ago, the boy and I were talking and he brought up something about a friends-with-benefits type of relationship and he said he still really likes me. I told him I'd be all right with trying it (I've never been in a relationship like that before, though) because I still like him.
And then today I found out he has a new girlfriend.

Fhgjhgkj. I just don't get why he would say that to me and then get a girlfriend. And I know for a fact they just started going out about two days ago, because I happened to notice a comment that one of his friends left him on Myspace.
Plus, I pretty much could have lost my best friend because of the whole thing.

I'm sorry, that was kind of long.
But, can anyone explain to me why he would do that? If he doesn't like me or doesn't want to take our friendship//relationship further, then why would he say he does? And if he does "still really like me", why would he get a girlfriend?(I guess he could just like her more)