I think it has to do with trying to process all the various emotions attendant with social occasions. Socialising is exhausting, and if I haven't had a particularly good time or have felt excluded I definitely have moments on my own where I curse myself for not being able to act normal or for looking weird. There's a couple of things going on, the first is that there is just a sensory and emotional overload that needs to release itself. The second is that often, in a large group of people or highly social event, as the night goes on I will start to feel more and more sad. It's as you say, a "tug at the heart," or like something is missing that I can't quite put my finger on. It's like I'm on the outside looking in at all of these people enjoying their time together, and I just can't relate or feel that same connection. The best word to describe it is melancholy. I cry not because I want to have lots of friends and be super social, but because the whole situation, that disconnect, is just sad.