Because cats are mental.
This morning after the dogs went out, Lucky came in. I wasn't quite ready to be awake, so I crawled back under the covers. Lucky climbed right up after me, snuggled in beside me and lay his head in the crook of my arm. Since it wasn't cold outside, I chose to think that he really wanted to be with me. Cats have a way of making you feel like you've won the lottery if they hang out with you. I did all the appropriate scratching of his chin, tugged gently on the scruff of his neck, let him rub his face across my fingernails, and all the other things cats like. He was, after all, royalty in my presence. I'd better do the job right. I watched his tail for signs of discontent. None shown. My cat really likes me!
About 20 minutes after this lovefest began, my arm started to go numb. He was laying on the main artery in my armpit. I simply wanted to move to one side, so that I could get some relief. I would continue my duties as stooge to the king once feeling returned to my arm and hand. My reward----he bit me! Not once, but 3 times! He didn't break the skin, but he was clearly annoyed with my slacking off on my duties. I pulled a sheet up between us to stop further beatings from my king kitty. When I pulled the sheet back down, he hissed at me and did his best cat king growl. I decided that his proximity to my face warranted his booting off my bed in short order. He landed on the floor and gave me the "What'd I do?" look. I dethroned the king.
He does this with Pearl quite often. She'll tell me about it like it's no big thing. He never breaks the skin. I just want to know what's going on in his head. His tail never gives any warning. The tail is the gold standard on cat emotion! In short, I believe that cats are mental. That is why I don't own a lion. Something about a lion giving me a few warning bites makes me a little wary.