I was just in the same situtation as you. this chick was giving me ALL kinds of signals, even tho she had a bf, her roomate even asked why I msg'ed her and said, "aren't you and her talking?" - i totally thought it was mine for the taking. however, just the other day we were talking on the phone, she asked me if i had a GF, and thought it would be awkward if i did and we were talking? but i was like, well, you have a bf, it isn't awkward? needless to say, after talking about it a little more, she basically told me that she didn't kiss me cus she has a bf, and isn't going to cheat on him.
sorry for going off on a limb....anyway, i think in short, they just like the attention.