Inbreeding is bad. It increases the frequency of homozygosity for deleterious traits.
There’s this little thing called pleiotropy. Selection is a powerful tool, but traits can have multiple effects, and extreme selection for peculiarities can have unpleasant side effects — you may think a pug’s curly tail is adorable, but it comes with all kinds of spinal ailments. And cute little doggies with cute little heads may have skulls too small for their brains, leading to syringomyelia.
If you’ve got an hour, this video is worth watching. Add pedigree dog shows to puppy mills as examples of animal abuse. Warning: there are scenes of dogs in extreme pain and distress here; not because anyone is directly harming them, but entirely because they’ve inherited a suite of damaging genetic characters that make their lives a misery.
The most appalling parts of the documentary are the responsible people behind the dog shows and the kennel club breeding programs that arbitrarily set ludicrous standards for show dogs. There’s a judge declaring that the German Shepherds with the weakened, ataxic hindquarters of their ideal is genetically superior, for instance. And then there are the photos of what dachshunds, beagles, and boxers looked like in the 19th century compared to the show dog ideal of the 20th — in just a little over a hundred years, we’ve bred this poor animals into a monstrous state.