I was thinking about this today - in relation to two separate instances.
First instance - there are two girls that I see in passing every single day. Basically, for the last several months I see these same two girls just about every single day during my break from work. For the first couple of months, it was a lot of "back and forth", being funny, semi-flirting, getting to know each other's personalities, blah, blah, blah. They are now my friends and they know it. But, now that I know quite a lot about them and the friendship is officially "established", I don't feel the need to engage them in conversation every day. Sometimes during my break, I would rather read the newspaper or sit peacefully, or maybe even talk to someone else. So, I'll walk into the room, see them in the corner and say, "hey guys!" and promptly pick up the newspaper and start reading an article. I've noticed lately that they seem sort of "offended" by this. They are both extroverts (which may or may not have anything to do with it - I think it probably does). I think they are getting this impression of me now - that I can be snobbish or give them the cold shoulder. They are my friends just as much now as they've always been. It's just that if I don't have anything in particular to talk about (or some new dialogue that we haven't had yet), I'm not going to just go through the same things we talked about yesterday just to make conversation. In my mind I'm not being rude, but I just think "Oh there they are. Hmmmm, what's in the paper today?" And then they seem to feel ignored. Basically, as the friendships have progressed, they are expecting MORE out of me, but I'm expecting LESS out of them. They're ALREADY my friends. We don't need to talk about silly introductory things anymore.
Second instance - I have an ex (more of a friend these days) who I now live "farther away from" (as mentioned in the OP). We are still friends and we still maintain contact but, well, it's not that I don't "need" her as much anymore - it's just, "what would you like to talk about" kind of thing?
That's another thing with NT's I think. The MORE we get to know people and the more we find out about them, there becomes less to find out about them. I'm very curious when I first meet someone. I want to know a lot about them (it's the "sx" variant, in my case). If I'm "not that interested", then it's just casual conversation. But, if I'm interested in the person or intrigued (not necessarily romantically, but just find them "interesting"), then I'm able to connect very deeply and form a very deep-rooted bond in a short period of time. It can happen very quickly with certain people (it's somewhat rare). But, after that initial stage, what else is there to talk about? Sure, we're friends and we will definitely have things to discuss, but, we don't really have to talk for hours. We can if you want, but it's not really necessary, because we've already discussed 95% of it. Most everything else is just silly stuff, like:
"hey, it's a hot one out there today, eh? Think it'll ever cool down?"
"Ummm, I don't know. How in the heck would I know? Do I look like a meteorologist?"
Meh! I'd prefer to read the newspaper! It doesn't ask goofy questions.