How come men will look at me but won't approach?

I'm a 27 year old woman... I'm tall and slim and people say I'm attractive... Whenever Im at the gym and look horrible (no makeup, sweaty, red) I catch a couple of the personal trainers working there checking me out... I dont get it? They never approach me but they keep looking... One was working reception the other week, when I signed in he smiled and was like, "hi how are ya?"... Was he just being friendly or has he been waiting for an opportunity to strike up conversation?