Norm Geras joins the psychologists explaining "one nagging thing they still don't understand about themselves":

I don't understand why small things - small bad things, small good things - can make me feel either down in the dumps or cheerful about life, even knowing that they're only small and don't really define the basics of well-being. You get something awkward to deal with in the post. Or someone pays you a compliment. Multiply by two or three on either side and it can take the shine off your day or give it a real lift, even though in each case it's quite trivial. I don't understand why knowing this doesn't change it. And I don't understand why if I'm feeling down in any low-level way of this sort, just doing something usually takes care of it. It can have that effect even when the original cause of the down feeling bears no relation to the thing I do in order to start feeling different.

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