I was actually thinking about it today. Here's an awkward truth about friendships that everyone needs to get comfortable with: often one of you is more interested in the relationship than the other. You know what I mean. For instance, I was invited to a wedding a few weeks ago. It never occurred to me to write to this buddy in many years, I didn't invite him to my wedding, but he invited me to his. I had a great time, we caught up and had a good talk. The same has happened the other way round, I'm sure. People I like a lot and contact, but they don't contact me. Yet when we hang out, everyone has a good time.
If you act weird about these relationships, you lose them. You don't want to do that, because marginal relationships are maybe the most rewarding to maintain, IME. People doing different things to you in different places bring a lot more into your mix than the ones you see every day. There also tend to be many of these relationship-seeds, so your close friends will grow from some of them.