To purr, for a cat, is to express sarcasm.

"Oooh, you're rubbing my back... am I supposed to enjoy that? Do you really believe this is a source of pleasure for me? Here, why don't I push up against your hand so you can rub harder. Oh, NOW you're scratching my ears. Of course, I could never reach there, only being one of the most flexible of all mammals, and of course my claws are hopelessly inefficient scratching implements. I could never do as good a job as you are.”

“Oh, now you’ve decided to feed me. This stuff is sooooo delicious. What is this made of, anonymous horse fragments and meat syrup? Mmm, I’ll just scarf this down so that I may experience a wonderful episode of spastic vomiting later. That would just make me the happiest cat in the world. You dumb sap.”

Purring is feline sarcasm. That’s why they shred your hand so often right after, and why it sounds so much like growling.