I strongly feel horses are happier and healthier mentally, physically and emotionally if they are allowed to live the way God designed them - out in the open, not in a box. Horses are not like us. They don't like small cozy places. As prey animals, they like to be out in the open so they can move and see all around.
Horses who live out in a pasture 24/7 are usually healthier than stall boarded horses who live in a hot, musty barn full of urine and manure odors. Walk into most full boarding barns at 6:00 in the morning when the barn has been closed up all night and the ammonia smell from the night's urine will knock you down. I feel sorry for the horses having to stand in there and breathe that all night. No wonder they develop respiratory problems.
I understand that not everyone has the option of pasture or paddock boarding and must board their horse in a stall with little or no turn-out. I also understand that stall boarding is sometimes necessary for some stallions, horses recuperating from certain injuries or for upper level show or performance horses that need the protection of a stall to prevent injury or coat fading.
In Wyoming my horses lived in the pasture and had a run in shed available to them but they never used it. They were happy to stand out in 15 below zero weather in a blizzard munching hay. They would grow a heavy winter coat and snow would pile up on their backs. As long as they had plenty of good quality hay their body would generate enough heat to keep them comfortable. (Note: if they were losing body heat the snow on their backs would melt.)
Observing my own horses in Wyoming indicates to me they obviously prefer to live outdoors. Certainly we prefer to be indoors on a cold or rainy day cuddled up by the fire in a warm blanket drinking a cup of hot chocolate, however my opinion is, we should allow horses to live in the way they prefer rather than try to force our human characteristics on them. Let them be horses and live outdoors!