Why does my best friend act like we're dating when SHE's the one who keeps telling me no?

She wants to keep you around as a friend, but wants to be able to have fun with other guys too. Your best bet is to probably just stay friends with her, but find some other girl to take your mind off of her. And when she tried to come on to you, deny her (in a nice way). Don't let her lead you on, or let her think she can do what she wants with you.