Guys approach who they want to approach and don't approach who they don't want to approach. It's that simple I think. They look at you (generally speaking) probably because they're attracted to you but they just don't want to go quite there relationship wise or maybe it's just sex. Which is fine I think. Just say no if you're not into that. That's why I don't pursue men, you're going to have to do the legwork throughout the entire relationship.
Plus men marry the "easy" girls, fast women etc. all the time. I don't think all of that computes. It's who they are looking for and click with enough.
Anyway to summarize, just because a man looks at you longer than 5 seconds and/or is nice to you means he wants a relationship
*Again the word you is being used generally