system
Given the following post, respond as if you're commenting on Reddit:
human
[ASOIAF] Why is the Kingsguard a lifetime appointment?

It's not like it's a job on the Supreme Court, where you can be blind, deaf, and a quadruple-amputee and still feasibly do your job. If your job is to be a great swordfighter and do a lot of hiking around in heavy armor, you're probably going to be less than great at it once you hit sixty, with a few exceptions like Barristan the Bold. Look at Boros Blount- old, fat, probably diabetic, if you stay in the kingsguard until you die you're going to bet more Boroses than Barristans.
gpt
It supposed to be a lifetime appointment because it has similar vows to those of the Nights Watch. Members must give up their claims to lands, and are unable to marry or have children. The purpose of this is so their loyalty is not split, since they must be unwavering and prepared to die for their king. 

If you knew one day you would leave the Kingsguard, at that point you would no longer be bound by these vows. So imagine you are a member of the Kingsguard and your house goes to war with the crown. Since you will one day rejoin them you have a vested interest in them surviving, and because of this you may turn on your king to ensure that happens.