to Cap'n (or tipple of choice). I reckon it's a combination of sheer bloody mindedness, proving the vet wrong, and the devoted care and indulgence by his slave.
My dad was like that - was told he had max 6 months, remembered that the bible (which he didn't believe in) said 3 score years and 10, so he hung on well over a year until he'd passed his 70th birthday.