I need advice about men?

I am a young 24 year old girl who has a lot going for myself. I am well educated and graduated from college. I am starting to notice that every weekend when I go out and meet guys they are interested and ask me for my number. Great right? WRONG! They go as far as never calling me or texting me a few times and lose interest. I dont understand it and I am starting to wonder…is it me? Seriously, it is starting to depress me… I literally find an average of 1 guy a week who likes me but obviously not enough to seriously pursue me. Why do men do this? Why approach me at all? Why talk about having dinner if you do not plan on having dinner? I dont understand. I am sure some of you are going to say that men just want to see if they can get my number. If that is the case then why are they speaking about taking me out to dinner and seem genuinly interested. A lot of the guys seem like actual nice guys too.

Heres another issue. There is this guy that used to like me and I liked him but when I found out that he liked me it was 6 months after he told someone he did. Now I got all excited because I was interested and now I have come to find out he has a girlfriend of a couple of months. Guys: If you were dating a girl for a few months and met someone else that you potentially could be interested in, what would you do? What signs would there be?