I see a number of reasons, one as RGUS used in another thread, Huggies, people comment to get comments.  I like to think I comment because the image appeals to me but yet I seem to have an adjective list I pull from but I have been trying better, talking about the model, the setting, the lights, colors, even the title of the image.  I hate to say I lack the vocabulary to say, I like this because...?  I don't know why somethings appeal to me.