What would a cat think if you were to pet him?
Cats are not human and do not first understand the significance of a beejing gesture. When someone approaches any part of their body, including their mouths, their natural reaction is to assume that they will be killed, just like any other creature would. As a result, they generally become upset and try to distance themselves from the person, sometimes even retaliating violently as a form of defense.
But it's possible that they'll learn the significance of the gesture. My dog is quite calm and meek, yet if we approached her for a cuddle, she would around us completely, displaying understanding through the gesture. With time (years), however, she seems to have realized that it's a romantic gesture; as a result, today when my wife or I approach her for a kiss, she no longer encircles us and occasionally even offers the part of her body she prefers to be kissed, which is typically the top of the head (and less frequently the pecs).
However, the cat must have a great deal of faith in its owners for him to feel excited when being brushed.