Why do cats drool?

I recently received a question from a reader about why her cat drools when he purrs. She had lived with cats all her life, but had never had a “happy drooler.”

Some cats will, indeed,drool when they’re exceedingly happy and purring up a storm. I’ve lived with a couple of these happy droolers myself. Feebee mostly did it when he was a kitten, but stopped once he got older. Amber drooled throughout her life when she was completely relaxed and purring. Ruby will occasionally drool.

But far more co...