I guess most people are enjoy seeing comments on their images. Otherwise how do you know if people liked your work. Yes, it's most important that you liked and enjoyed your work yourself, but appreciation of other artists is not less important. So if someone spend time and after visiting my image left comment, I prefer to spend same time to browsing this artist work and leaving comment, showing that his work creates response in others, rather spending same time typing "thank you" note. Not many of us has unlimited time to spend on Pbase. So with my limited time I'd better make other person feel better by noting his work and leaving comment. I do send a "thank you" note if someone spent time and browsed the entire gallery, and particularly left critique comment. It's just an appreciation of time spent by the person. And of course there are comments left, in anticipation of reciprocal comment. But no body forces me to leave comment under person's image. If I like it I do, if not, then I won't.
My 0.02
Michael.