It depends on the clothes, the pet, and the weather. If you live in excessively cold area, I understand putting shoe or boots on you pet's feet to reduce frostbite. I also understand when owners will give them a vest or a coat to keep them warm. The only time I get really concerned about animals wearing clothes in hot weather. I've seen a labrador whose owner liked to not only dress him up in clothes but in layers even if it is over a hundred degrees out. That is where I think it's cruel. If it's hot out or extremely humid, animals shouldn't be forced to wear clothes unless they have a health condition where they have trouble regulating their temperature. On the movie, Legally Blonde, the main character's dog used to like wearing a tutu. I don't mind that because most of the fur wasn't covered so it wouldn't interfere with sweating in the hot weather and made the dog happy.
