Flickr: Oliver H
Not all cats drool when you pet them, but some cats are big slobbers. They drool when they feel relaxed and content or happy. This drooling behavior is still a mystery as to why some cats do it, but it is more of a conditional reaction such as when we are hungry, we drool. When certain areas of a cat such as the back, neck or chin are stimulated, it triggers the cat to salivate.
For many cats, drooling takes place when they are sitting comfortably on your lap and enjoying the wonderful massage you are giving them. They feel so content that their muscles relax, causing their mouth to open, dripping drool and purring away. They show similar reaction when they experience catnip which provides euphoria that causes some felines to drool [See Does Catnip Get Cats High?]
(Play the video below to see a cat drool while getting a massage)
You can find out which areas may trigger your cats' drooling behavior by experimenting petting their head, chin and neck and see how they react to your petting. Once you have located the trigger spots, you can restrict yourself from touching those areas. Instead you can try to speak in a soft, soothing tone to comfort them. However, drooling is a very difficult behavior to correct, if your cat drools a lot, keep a washcloth within reach.
There are cases where drooling can be medical. If so, you should check with a veterinarian about your cat's drooling problem.