Years ago I had a boxer that went downhill virtually overnight. He was a big gentle giant weighing in at about 95lbs. He had a routine checkup on Monday with no issues. Come the following Thursday, he fell down while my then wife was walking him. A trip to the vet revealed calcium deposits on his spine, something that would not be normally routinely checked unless warrented.
If I recall correctly, the vet put him on steroids (ugh). Saturday morning, I came downstairs to see a big puddle (due to the steroids). I let him out while I cleaned the floor. When I went to check on him, I caught him as he fell down. By the time I got to where he was at the end of the yard, he was gasping his last breath. He died in my arms. I think that he was ten years old.
We dropped him off at the vet's to be cremated, not being able to bury him in our yard at the time. A friend who happened to be a boxer breeder offered us the last of her most recent brood. So, after dropping him off, we continued on to pick up the new pup. That was the quickest that I had ever brought a new dog into the home.
Other than that, the only shocker over loosing a dog was when our rottie went in for surgury and was found to have cancer. We decided the best course was to not "wake" her up. That did not make burying her any easier for me though.
We have seven rescues at the moment, three purebreds and four Heinz 57's. We had to put number eight down a month or so ago. While we miss her, the others keep us busy enough to not linger on the loss.
We also have horses so having the vet out is routine. For the last two, we were able to line up the vet to swing by rather than to take the animals to the office then bury them on our farm. We have a quiet "glade like" area set aside containing several animals, large and small. When I mow and trim, there is always a flood of memories to contend with.
Part of being a good pet owner is knowing when to let go for the good of the animal no matter how much it hurts.