This is an interesting video from a couple months ago. It talks about the behavior of the pupils during certain emotions and cogitation. When you're physically attracted to someone - your pupils will dilate. Parts of the other person's subconscious will somehow detect this and also dilate. This sets up a nonverbal positive feedback loop of which we're not aware of consciously. Demonstrated 40 years ago, there is still a big mystery here. Although we know this absolutely takes place - we can't explain how. It's extraordinary to think the subconsciousnesses of two people are communicating ... and collaborating without their respective consciousnesses "approving" or even being aware. Via this pupillary feedback loop - attraction begets attraction. Thus, even if initially you don't find someone necessarily sexually appealing, after an encounter you may find yourself drawn to them and not know why. This is one body language mechanism which explains this phenomenon.