I do feel that it is important to consider each pet and their needs, though. A very young or old pet is more susceptible to ambient temperature. An arthritic pet needs a comfortable place to lay down and care must be taken going up and down stairs. A short-coated or hairless pet needs some coverage in cooler climates!
Years ago, I had a much beloved Greyhound, Lotsie, and living in a cold winter climate, her fleece-lined jacket was essential for her comfort on walks and outside time. This is true for many pets; clothes are not simply for fun, they are necessary in some climates.
Summer may also necessitate "clothes" for pets, such as life jackets for dogs. Not all dogs are good swimmers. Additionally, if your pet is knocked off of the boat (perhaps getting injured in the process), or is tired/cold from choppy water or sudden storm, a life jacket could be what saves your pet's life. Even though she is athletic, my dog Sophie wears a life jacket for extended times in the water.
Things to look for when considering pet clothes
Safety and comfort are the most important things to consider, in my opinion. Making sure that the coat fits properly, snug enough so a foot or mouth won't get caught where it doesn't belong, and loose enough so it isn't constricting. Also consider how easy or difficult the item is to put on and take off. If your pet is caught or injured, will the coat be easy to remove quickly? The material should be rugged and weatherproof. Loosely woven items can easily catch a toe or claw, creating a problem.
Caution is also advised when leaving your pet unattended with clothes on. That may sound humorous, but... if your pet is bored or doesn't like the clothing, chewing and destruction may be the result. It is easy to think of your pet sitting there, comfortably by you, everything fine. But what if your pet escapes and is running loose? Or gets the clothing caught on the fence while trying to escape?
Bottom line: while I am not generally in favor of clothes for pets "just because", I do feel that jackets and outerwear do have a purpose, depending on the pet and the climate.
Related Reading:
Shop for dog coats
Shop for dog life jackets
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