Because the sky so high, the bird shit in your eye, and your kar
chng y y... 
But seriously, almost every guy I know goes through this kinda
situation at some early point of their love life and come out of it
wiser. The answer is quite simple really. You're not the guy for
her, but she enjoys the attention and care you're giving her - put
bluntly, you're playing the role of sucker. No offense, just coming
straight to the point.