For me, pain has been the deciding factor. If the things he enjoys become painful, or even his life in general, then it's time to let go. I had to fight with my vet about letting Loca go. She didn't want to, I knew I had to. She didn't know Loca or her zest for life. I also know a lot of people thought I let Phoenix go to soon, but I know I didn't. Life can't be fun when you're in pain.