Another thing I'm sure of is how little time most people spend thinking about other people. Even the people in my life I don't care for, I rarely spend more than a passing moment thinking about them, even if they're in the same room.
The ones I do think about a lot are the ones I love (wife, children, closest friends).
Despite knowing this, though, I'll admit that I can spend a lot of time thinking about That One Dumb Thing I Said at Dinner Around My Friends - again, though, I'm 99% sure most people don't remember the dumb or embarrassing things others say or do.