I just stopped talking to them or responding well to their efforts. It's a trend. I really couldn't even tell you why with any absolute certainty, aside from the following thought that's come up when trying to figure it out.

If you grow up in a situation where your parents move every couple of years for work, IMO you're going to develop in one of two ways:
-you're going to get really good at making new friends, real fast, and keeping in touch with people over time
-you'll reach a point where you stop putting any effort into connecting with new people or keeping in touch with old friends, because what's the point? You'll be gone soon anyway.

And if you're in the latter camp, unless you put real effort into fixing it, that shit can stick with you long after the situation creating that condition is over.

I've made some progress, I suppose, in trying to at least be a friendly guy on the street open to chance encounters that theoretically could turn into a more robust friendship, but I've got a ways to go to get where I'd like to be re: that.