Just the IDEA that there is a better adversary out there, or people that are unpredictable, or perhaps the knowledge that you WILL age can also motivate you to improve your skills.
Whatever the reason, there needs to be a problem to solve and one of the greatest skills to have is to find problems worth solving.
Unfortunately the ego is an impediment to this ... it does not like putting itself in uncomfortable and novel situations, and will convince you that it has found some great problems when in fact it has set itself up with familiarity and easily solvable crap to confirm what it already knows .....
Play with people higher skilled than you and learn how to beat them.
Play with people outside your system, outside your range of experience, and see what comes up. Basically find out what you haven't even thought about, because if you look within your comfort zone, you aren't even going to SEE the problems ....
You just don't know what you don't know ..... so best find out.
Here is a rather dense piece by John Boyd, yes the same one that came up with the OODA loop, about reality, concepts, problem solving, and thinking outside the box (pdf format).
Thanks yet again to Steve Morris for sharing: