And that, which I thought was going to be a digression, leads right into what I was intending to talk about. A number of non-science-fiction writers have pointed out that your brain, which sometimes can't remember if your child was with you when you started shopping, nevertheless retains embarrassing memories from early grade school with full fidelity. [Dave Barry] It clings to them — every misunderstanding, every time you were a total doofus, all the dumb things you've ever done — and loves to drag them out and punish you with them over and over again. In many cases, they're things that nobody else even remembers, or cares about if they do, but you'll remember them until your dying day.
I'm only 40, and I've led a pretty untraumatic life, and even so sometimes it's hard to find anything to think of which doesn't drag up a bad association. Imagine having that accumulating for centuries, or millenia! As noted above, immortal humans are liable to get pretty peculiar.