Confused

A guy i used to see is dating my friend but she doesnt know i dated him. When i was at her birthday anytime i walked past him he would pull her closer to him. Why would he do that?

How come you didn't tell your friend that you used to 'see' the guy? And by 'see' I'm assuming you mean sex...

I don't know why he would pull her closer to him when you walked by, I would think it was an involuntary reaction because he is probably afraid you may interfere with his current relationship.

It could be a coincidence and you happened to be hyper-focusing on them, or it could have been a childish attempt to make you jealous.