I was once on a flight, sitting in row one which had a huge amount of leg room. The man behind me was blind and had a beautiful service dog which squeezed itself under the seat in front of the man. I felt sorry for the dog and changed seats with the man so that the dog could have the big empty space in front of his new seat. After we switched seats, the dog again crawled under the seat, just as he had been trained to do.
The man was flying from SEA to LAX to turn in his existing dog and train to live with a new one. He said that they are only allowed to have a dog for ten years and his new one would be his fourth. It made me sad to think of him parting with his beautiful dog. He said when the dogs retire, they are adopted by families. Lucky families.
For years I flew back and forth weekly between SEA and PDX, which is a route they use for training service dogs to fly. Even the service dogs in training are well behave by the time they get the stage where they are learning how to fly.