Isn't it a great feeling when someone takes the time to write a helpful, supportive or event positive yet critical comment on an image you've taken? Not only does it generate a nice warm feeling that someone has taken the time to comment but also provides useful feedback to the photographer.
Although I haven't received too many comments on images I know how great it can be when I do get it and will undertake to post a comment on 3 or more images every week in an effort to spread a little cheer to other photographers.