What makes people flock to each other?

So this week, I was at a work event and we had a good mix of people there. There were probably about 80 people in attendance. I walk in and look around and oh my goodness! it feels somewhat crowded. Maybe it's because people are all by the bar. I take a quick glance around and I end up moving with my sub group to a less crowded section (I've learned that I dont do crowds too well). Anyway, we get settled in and begin to eat dinner and I gradually begin to expand beyond my group. I'll say hi, to the person next to me and talk to whoever I'm introduced to.

Then I had someone walk across the room just to say hi and she was actually pretty nice, we introduced ourselves and talked about what we were working on and we ended up diverting to a different conversation entirely.. travel, shopping and places visited. I have to admit that I think it was a really bold move on her path and I sat wondering huh? What made her come by to say hi to me? Was it because I looked happy? friendly? She just wanted to meet people? I couldn't think of any solid reason. I am very glad that she did come by and I could only imagine how many people I am in the same room with that I dont get to talk to because they are not in close proximity to me that I could probably find a lot of things in common with or share ideas with or something.

So I started to think, how did I meet most of my good friends? We were introduced, we took classed together, shared a joke while in close proximity to each other, share a passion together. I dont think I've ever introduced myself to someone just because.... Anyway, I'm learning that when I do go out to any event, I would TRY to get out of my element and comfort zone to actually meet people. I only wonder what my criteria for choosing who I should meet would be.