I was a funeral director. I talked to a lot of people about death and I can't recall a single old person who wasn't alright with dying.
The way some have explained it to me was basically the novelty of life was gone. When you're young, everything you do is new. You experience things for the first time daily. As you get older, new experiences are far and few in between. Every day becomes a copy of the day before. Weeks pass where it seems nothing has happened, but its only that nothing new has happened. Old people eventually just get bored. Combine that with chronic pain, being unable to do basic things on your own anymore, and all your friends dying before you, and the idea of not waking up tomorrow doesn't seem that bad.