I have to agree on this. I think one of the reasons is, at least for me, that you are not sure the pet understands what is happening or why. Two of the ones that hit me the hardest were first my Shepard Husky mix, Harley. Had him since he was a little pup. When first wife and I got divorced he stayed with me and supported me big time through that period of time! After I met second wife she came with a Australian Shepard. The two of them hit it off great then at one point Harley went down hill bad and started having problems and could not control his bowels and you could tell he felt bad about making a mess. When we took him to the vet to be put down as the vet gave him the shot he clamped down hard on my thumb, it was sore for several days always felt it was his way of saying by! Second one that hit me hard was Libby the Greyhound, we more or less inherited her from her owner who was a Veteran for whom she was a service dog to help him with PTSD and stability problems do to injuries. He got to the point he could not adequately care for the dog and as we had been Libby's foster home when he had to go in the hospital on a couple occasions he asked if we would consider adopting her. That was a no brainier as she was not only the sweetest dog ever she also was extremely well trained. Then one day I took her out to the pen to do her business and like often she took off to run a bit first. Flat out diagonally across the 40X50 foot pen and did a 180 to come back and she let out a scream of pain and just barely was able to limp back to me. Long story short turned out she was dealing with bone cancer and that quick move shattered one of her rear legs. To see her go from being happy and full of energy one second to extreme pain while I stood there was hard to take.