John never wanted a cat. He's more of a dog person, but he never wanted a pet in general. Because when you have pets, things like this happen. Things like waking up to a grisly crunching. Lying still, hoping he's dreaming, then opening his eyes to see, horror of horrors, a mouse spleen on his pillow.
For some reason, this never happens to Sherlock. Debbie never gives him 'gifts' of desiccated birds and torn-to-shreds grass snakes. (In his opinion, cat 'gifts' are rather like dick pics; flattering, maybe, but not something he particularly craves.)
Not for the first time, he curses himself for not giving the little tabby kitten to the RSPCA when his flat-mate first brought her home.