Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic depicts the universally relatable experience of intrusive thoughts about past embarrassments. In the first panel, a man is walking along happily, whistling to himself. Then a message appears, formatted like a letter from "Your Brain," cheerfully reminding him: "Hey! Remember that thing you did once that you're ashamed of now?" In the final panel, the man is shown walking with a dejected, miserable expression, his good mood completely destroyed.

The humor comes from personifying the brain as an almost malicious correspondent that deliberately interrupts moments of contentment with cringe-worthy memories. This is a near-universal human experience -- the way our brains seem to ambush us with embarrassing memories at the most random and peaceful moments. The letter format makes it feel even more intentional, as though your brain is specifically choosing the worst possible time to remind you of your past mistakes, not to help you learn from them, but simply to ruin your day.