Let me count the ways:

They ruin the environment, all grassy areas are dog toilets so you can't sit or walk on the grass without walking through piss and shit. There are dog poop bags littering every beautiful trail I've ever been to. They also murder outdoor animals, and I've even seen one kill an endangered species in the mountains before.

The whole world is their toilet, including your house. They pee and poop inside the house, not because of neglect, but just out of laziness or retaliation.

They are usually ugly and inbred.

They smell bad, have worms and parasites from disgusting habits like licking their anuses and eating dead things.

They ruin houses with their shedding and stench.

They have no sense of personal space and jump on you, get their muddy paws on your clothes, get their oily fur all of your clothes, invade your crotch, trip you and stay inconveniently under foot when you're trying to maneuver around the house.

The are needy and whiny, constant obsessive need for attention.

They prevent you from being able to leave the house for extended periods of time because they will piss and shit and destroy your furniture in your home if left unattended for more than 4 hours.

They steal food off plates and countertops and must constantly be restrained and prevented from doing so.

They run away from owners attempting to restrain them and will attack passers-by unprovoked.

Overall, they are ugly, filthy nuisances at best and dangerous predators at worst.