I never understood their motives, either. People see one video they don't like, so they go through all my videos to leave some bland insult that doesn't bother me because internet insults are meaningless. What's funny is that when I still read my YouTube comments (after a while I just let my account sit there and didn't pay much attention to it) they were usually something like "DIS GUY HAZ NO LIF!!," and still I never went and left the same comment on all of their videos.
I don't know why I'm posting this, though. It's pretty common knowledge that YouTube is just one big cluster of every person with an IQ below 70, plus a few people who will grow up to do more than work at a convenience store.