Wit of the Staircase

l'esprit de l'escalier n. Fre. "Those biting ripostes that are thought of just seconds too late, on the way down the stairs—or even, to tell the truth, days later." As coined by Diderot, best known as author of the original Encyclopedia. Literally, "the wit of the staircase."

This perfectly describes, well, everything. How many times have I stood flabbergasted in debate, or even in simple conversation, feeling vaguely insulted but having nothing to respond. And then it strikes, in the shower, that "perfect biting repost." Why does my mind work on just a lag?