I like to think of it as having provided a comprehensive reply to the OP such that no one else thinks they can add more to the excellent post that then ends up being the last reply.
Ah...I see what might happen here.....
In the meantime we could discuss the topic - maybe we always want there to be a reply to the threads we post in, just to maintain the pretence of a conversation, human contact, answers to our questions etc. and if there are none - we feel that much more than the threads we post in and receive our answer.
Therefore we only remember the unanswered questions, because they make us feel more alone or ignored. THAT'S IT! We always want a reply to feed our own self importance, to secure our place in history with an indelible (unless by moderator or forum glitch, natch) and public record of conversations and thoughts that can be read by all and sundry the world over. We exist because someone else has read our typed thoughts and chosen to respond. If no response, maybe we die a little inside and retreat into our shells a bit more. Right now I can remember many posts where I have had the last word - there is no triumph in those memories, only failiure.
(Sub, just trying out your theory...)
"...and the monkey pulls the switch!"
That's actually a very reasonable theory.
I think electronic communications in general have a certain level of addictiveness built into their use, such that we get a tiny high off of others replying to us, via email, txt, post, tweet, etc. We crave that reply, and wait for it, like an addict waiting for a hit. But once we get it, we just want the next.
Anyone who has been bored and sent a few txts to friends, only to wait forever for replies knows what I'm talking about. The ping! of your phone going off is the fix you want.
We're all screwed up, guys. :shrug: