It's funny you post this, because I was thinking about the same thing this morning. I have run into a number of different people who have upvoted me and then sent me messages whining when I didn't upvote them back. It always makes me think... uhh, this isn't a mutual back scratching service; I upvote things I like and I am under no obligation to return any upvotes I get. I think either my idea of how upvoting works (we vote for things we like) is different from how other people see it (which seems to be as a quid pro quo thing as we both try to profit) or I seem to attract the angry voters who expect this. Hmm... Anyway, good to see it's not just me who gets this kind of thing from people.