Why does everything get lemon or lime at the end?

Rich food — meat, cheese, avocado, anything fried or buttery — coats your tongue. A few drops of acid cut through that coating, so the next bite tastes as vivid as the first. Without acid, rich food starts delicious and finishes heavy.

Acid also does for brightness what salt does for volume. If a dish tastes flat and salt doesn't fix it, acid almost always does. This is the two-question diagnostic that fixes most 'meh' dinners: does it need salt? does it need acid?

Acid is why the chicken bowl gets lime, the lentil soup gets lemon, the salad gets vinaigrette, and the pan sauce gets a squeeze at the end. It's usually added late because heat dulls its brightness.

Lemon, lime, and vinegar are mostly interchangeable in this role. Use what's there.