Because they're such incredible loving loyal companions, that's why.
Last weekend we took our boy Orion, who turns 9 in a few weeks, to the vet because of a limp that kept coming and going. Being a senior dog, we assumed "old age" was setting in and increased his glucosamine intake, but wanted him checked out anyway. The doctor informed us it was very likely that we were dealing with an aggressive bone cancer.
Today this was confirmed by a specialist.
The prognosis: 4 months or until either pain management ceases to be effective or his bone breaks (the cancer has already thinned it out). By the time this type of cancer (common in large breeds and prevalent in giant breeds) is detected, it has already spread throughout the animal.
Any of the available options are strictly pain management options.
At his age, having lived a fairly long (for a dog his size) and very happy life, we cannot imagine putting him through a leg amputation or extreme radiation treatments. This would be selfish on our part and not change the prognosis in any way.
The plan for now is to keep him on pain medication and spoil him rotten for a few weeks.
Then allow him to exit in the dignified manner he deserves...before unmanageable pain sets in.
Like with the other pets we've had to "put down" in the past, I just can't imagine him gone.
He's the last man standing from our original pack (his mother Bella, our recently deceased St. Bernard Roxy and his brother Amadeus).
I seems only yesterday he was born in the bathroom of our first home.
Orion: 2 months old