The style of the previous post was not chosen purely for aesthetic purposes; I also wanted to abstract away from the details of the situation because this has happened to me more than once. One of the times it happened was yesterday, hence the timing of the post, but my concern is not so much with that situation (which is pretty stable and, indeed, comfortable) as with the general pattern.
My main impression of these experiences is that it sure seems odd that on multiple occasions girls that I find attractive will start talking to me and actively seeking to spend time with me and generally doing things that, to me, seem to indicate interest, only to casually indicate that no, they are definitely not interested. The conclusion I am increasingly drawing, however, is that these are really just friendly people doing what they consider normal, friendly things, and I'm the one who's reading too much into it.
The problem, then, is that I don't seem to be able to tell which signals indicate interest and which don't. I've long had the problem of assuming interest where there is none, but I had thought I had largely overcome it. I guess not.
(By the way, the comment thread to that post was great. Thanks to everyone who contributed.)