It may have to do with the manner you bring it up - it's not hard to see how saying something like "X is amazing" could be interpreted "X is amazing...and you're not" (after all, how often do you tell your friends how amazing they are?), in which case the bias is some combination of status jockeying, cognitive dissonance and ego protection.
Wow, that's seems like a very likely hypothesis that I completely missed. Is there some piece of knowledge you came in with or heuristic you used that I could have used to think up your hypothesis?
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July Part 1