I know I'm going to step on some toes with this one, and maybe it's already come up earlier in the thread, but...
#1 Owning pets in most cases. I can understand the purpose of service dogs, companion animals for lonely elders, etc. But otherwise pet ownership for the ostensible purpose of holding an animal captive to bring you joy* -- that's something I really have trouble wrapping my head around. The cost, smell, mess, time commitment, restriction on travel (gotta find airlines/hotels/etc. that can accommodate Fido and/or get a pet sitter), and so on...seem like a very steep price to pay for whatever benefit people feel they get from the pet.
*I do understand that some animals like dogs have a symbiotic evolutionary relationship with humans. The other points remain, however. Plus, I feel particularly uncharitable toward dogs/dog ownership simply due to the fact that other people's dogs are a recurring daily nuisance in my life. The barking, stepping on dog poop in my yard, the holes dug in my gardens, the dead spots in my grass from dog urine, and the fear of another family member being bit by a dog (again) has made me completely unsympathetic toward dogs or their thoughtless, lazy owners. I do acknowledge that some dog owners are conscientious about cleaning up after their dogs, keeping them on a leash while walking, not letting them bark endlessly, etc. I appreciate these people, and on the rare occasion I encounter one, I let them know it!
I've had pets before. When I was a kid we had multiple dogs, cats, birds, fish, chameleons, snakes, rats, mice, gerbils, hamsters, guinea pigs, rabbits, a tarantula, and I'm probably forgetting a few. As a young kid, it was a fun novelty to show off to my friends. But eventually I became at least peripherally aware of all the work involved, and the expense. I was acutely aware of certain aspects, like having to pick up dog poop in our yard every week (and step on it while playing in the yard), the stench of the catbox and the unpleasant daily task of cleaning it out, ALWAYS having cat hair all over my clothing, the annoying feeling of bird seed underfoot, and the embarrassment of having guests come over to our house and clearly reading on their faces the disgust at the smell of the various animals.
At some age I also started to feel that it was cruel to keep creatures in tiny little cages for our amusement. I was glad every time our bull snake escaped into the house, because at least it was free to move around and not sit in a little glass box waiting to be fed frozen baby mice. Watching any rodent run on a hamster wheel because it has nothing else to do or nowhere to go is really depressing to me. It's right up there with caged birds that sit on their perches all day and don't even know how to fly, or goldfish swimming in circles in a bowl.
Full disclosure: as a young adult I owned two cats. I grieved when one of them died, and didn't feel much when the other one died -- it was always half feral and didn't like us much anyway. Now that they are both dead and buried in the yard, I frequently thank them for reminding me what a pain in the butt pet ownership is.
#2 People who think of/talk about their pets as their 'children', and especially equate pet ownership with (human) child rearing. I've owned pets and raised children, and IMO this is complete nonsense, bordering on idiocy.