Do guys like sluts or good girls?

I have this guy friend and he is really nice to me, nicer than he is to any of the other girls at school, I just think its really weird, he's always flirting with all these really slutty girls, and when it comes to me he doesn't flirt, he's just plain nice and he seems really caring about me too.and he's known to be a player as well, he hooks up with multiple girls at parties and one time we were at the same party and he overheard some guys talking about how they wanted 'bang' me because apparently I was looking really hot and he started getting really overprotective, he comes to me when he needs somebody to talk to about his problems, am I supposed to feel used? because I'm always more than happy to help my friends out with their problems, always. I'm just confused, I feel like a little sister to him, and he always seems to be chasing after sluts, but I really like him.