I know it's superficial, but it still gives me a buzz to wake up in the morning and see a bunch of up votes and replies to something I wrote the night before. It's equally frustrating when I say something I think is smart and it gets ignored.
Damn you internet, validate my existence!
I don't think it's about validating your existence. I think it's that internet communities are gradually replacing more traditional forms of interaction. We, even introverts, still crave some amount of interaction; if we don't get useful interaction elsewhere, we can maybe get it online.
HN, and email, and everything like it becomes a lot less important to me if I've spent the day having fun with other people. (Counterpoint: I also wind down on HN after a stressful day of dealing with other people.)