Yeah, I think costly signalling is definitely part of it. I think there’s really several different things going on in the birthday example. One, the friend knows that you decided to spend the evening with them, so they can infer that you want to perform friendship, and/or anticipate having a good time with them, enough to make you decide that. This is the costly signalling part. But then there’s also the stuff that actually happens at the party: talking, laughing together, etc. I think this is what actually accounts for most of the “feeling closer”. (Or perhaps these two effects act on different levels of “feeling closer”).
Anyway this is maybe getting unnecessarily analytical.
Nod. FWIW, I was actually in part referring to costly-signal-to-yourself.
I also agree that there’s probably multiple different levels of feeling closer.