Why do some men like big rear ends?

I really don't know why....and also why do guys just stare at girls and I don't mean a "I like you stare" I mean a gaze o.o <---like that. then when that happens to me I usually smile and they don't even smile back or I will look for about 2 seconds and I'll look away and out the corner of my eye I can still see them looking....so if anyone knows please fill me in...