Why does a cat drool excessively?

There are several reasons why cats drool:

Some cats drool when they are really happy, relaxed, or even when they sleep; just like some people do.

Alternatively, a cat that is drooling excessively could either have been poisoned, or there is something wrong with its mouth (gum disease or a broken tooth can cause extreme pain to a cat). It is highly recommended to take the cat to the vet immediately, especially if the cat has lost its appetite and drooling for no obvious reason.