Note: This question was asked of me in FormSpring. I am just answering it.
I know that feeling, and I’ve been there. A lot of times it is just the company that you make. If you hang around with lots of couples, it’s going to be couples flirting with you, and they most likely have couple friends.
The other thing might be that they don’t really feel like they have anything to lose because they already have their man. They get to have a little fun and it can’t really go any further (usually). I’m sure that all happens subconsciously though.
Also, depending on where you live, there are just a lot of couples in the city that go out to the bars and are active in other ways. So your chances are just higher in that respect.
In closing though, relationships come and go. If you think a person is pretty cool, try to establish a non-sexual friendship and see what may happen in the future. At least you’ll come out of it with cool friend.