I have found that certain types of creative entrepreneurs and artists simply get extremely bored with any form of redundancy whatsoever. It can even cause them to go into depression. Facing new challenges is what motivates them to get out of bed. If they don't have a new direction to drive them, they feel like they are going backwards.
Then again, it may simply be that Steve is a perfectionist, and he realized at a certain point that his voice simply couldn't reproduce the notes that he was able to hit when he was younger.
Just thinking out loud.
