I used to hardly comment on stuff at all, but I do it more often these days - I mean, I know how happy I am when people comment on my stuff.
That said, I'm less likely to comment on something when they a.) are super-popular and already have tons of comments, or b.) I don't have anything to say other than "Aw, cute! <3".
It also depends on the time of day the comic updates/when I read the update - if it's at a time of day when I'm properly awake and alert, I'm more likely to comment. If it updates just before I go to bed, I probably won't, because I don't make much sense when I'm tired.