Ignorance

I'm talking about the act of being ignored, not someone who's stupid. Okay so heres the deal. There are several girls I know at the college. One girl who I talked to several times seemed really friendly, so I started sitting by her so we could chat and stuff...and dude to several other factors that I won't go into. So anyway she seemed pretty friendly and really nice, but then basically over the course of a day, the next time I came to class, she didn't even look at me. So what's up? I thought people out grew that kind of behavior after High School. I mean whatever, sure it sort of bothers me but I would just like to know why some people act this way.
Something else on that same line...this other girl in another class. I always sit on this one row...and it doesn't seem to matter where I sit, this girl always sits beside me...BUT she doesn't really ignore me at least...slightly sometimes but you know. Any explanations for these behaviors would be welcome.