My daughter has a dog that she dresses up in the winter and occasionally in the summer.
In winter, he wears a t-shirt and a fleece lined jacket with a hood and two pairs of dog boots. (sorry can't find a photo)
In summer on the really hot days, he wears sunglasses (called Doggles)
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The tourists in Whistler love it when they see him and he gets the 'papparazi' treatment very often as a result.
People who dress up their dogs (or dye their fur) are folks who love their mutts. There generally isn't any mental illness going on, just love.
Hooch wears the Doggles on days when the sun is shining so bright that any one of us would be reaching for our sunglasses so she's protecting his eyes from sun damage in this. As for his boots and fleecy jacket with a t-shirt under, well try taking a 65 pound dog for a walk in -14 degree temps (that's 6.8 degrees for the American folks here) and having him curl up in a ball, crying because his feet hurt so much from the cold - and then you have to carry him home!