It is a blight upon my soul and it's how I know I have one.
I've always had big dogs. MY last dog (I had her when I met my wife. She was 2 then) was a 90 pound Dobie-Rottweiler mix. Short haired, black and tan, and
the most athletic dog I've ever known. That dog would search a ten acre property for a tennis ball until she found it, then bring it back and drop it at my feet as her tongue hung to the ground and she put her front feet in her water dish to cool off. Then she'd pick it up and throw it at me to toss again. She would do anything I asked her to as long as that ball was around.
She saved my daughter from an attack by a neighbor's loose (normally chained) angry dog once.
Vet said to me when I got her ("nothing but trouble here - brittle Dobie bones, prone to illness"), but after 14 years, that damned dog never had so much as the sniffles. She got annual vet visits and never anything else. I fed her one brand of dry dog food - no human food.
6 months away from the end, she started leaking a little. She never exhibited pain or complained about it. Not once. 3 days prior, she wouldn't eat. Wouldn't even get up. I was working 60 hour weeks, but I laid on the floor with her every chance I got.
Third day, she got up and dragged herself outside to dump some rock-hard loads. She seemed to look back and apologize to me for pooping on the driveway, honest to goodness.
Took her up and the vet said thousands of dollars might get me 6 more months of her. She was simply shutting down.
I held her head in my hand and talked to her until she was gone. That was tough, but I didn't want to put her through all that.
I don't know that I could shoot an aged dog like that one.
Don't get me wrong - a rabid dog, or a mortally wounded (and untreatable) dog, I'd have no issue ending their misery.
It's tough, for sure.