How do you know you are doing just fine?
I'm pretty sure they just look at their bank accounts.
In all seriousness, there's doing fine and being greedy. There will always be a step to climb, a market to reach or revenue to grow. The goal is to do enough to enjoy life and be comfortable, not to be the richest man in the world. At some point, you can do "well enough" and not worry about what "could have been".