There's been a lot of stuff out there lately about this very topic, so here's my two cents worth. And this will be permanently posted on my sidebar (it's already there) in case anyone gets mad at me in the future for not commenting. Just so you're warned.
I write for personal enrichment. It forces me to use my brain, improve my vocabulary, focus my energies, and exercise my talents. Even if there is only one person out there besides my blood relatives who reads a word I've written, I want my writing to be as clean and polished as possible for that one person, and for myself -- because I am a bit of a perfectionist, and because I have found that it is a singular pleasure to go back to old posts and reminisce about what my kids were doing, or what I was thinking about. I am grateful that even though I can't remember what was happening in my life six months ago (precisely) I have recorded something of the thoughts and events I was experiencing then.
I also read for personal enrichment. Sometimes I comment, sometimes I don't. But I never (can I say it louder? NEVER) comment just because I want someone else to comment on my blog. I would call that insincere. I would call that fake. I would call that a bit too much like middle school for comfort.
I comment when I feel moved to comment; when I have some valid question, or an answer for someone else's; when I feel inspired by someone's post, be it hilarious or harrowing or heartfelt. But I don't not comment because I disliked something. Sometimes it's just the opposite, and I feel like anything I could say would seem trite next to the extremely wonderful post I've been reading.
I expect the same of you, dear readers! Don't comment on my blog just because you want my comments. You may or may not get them, and if you do, it will have nothing to do with reciprocity. The only thing that will get me to comment on your blog is content.
So. I write for me. I read for me. Sometimes I comment. Sometimes I don't. Sometimes I get comments. Sometimes I don't. But either way, I'll still be writing. And either way, I will have many happy days of reminiscing and remembering in the future because I had the wherewithal to write down some stuff about my life.