There's a lot of truth to this scene from Trainspotting, in which Sick Boy explains his unifying theory of life to his friend, Mark.
It's a very universal phenomenon: you get a bit of success (or maybe a lot of it) and then, as you get older and more comfortable, you tend to become complacent and less aware.
Maybe you don't push the envelope as much as you did before, or maybe you just get to a point where you're more easily satisfied and content to rest on your laurels. It doesn't have to be an age-specific thing either, it may just be a lack of enthusiasm or vitality. That old hunger you once felt is nearly gone.
Be cognizant of this reality and its tendency to creep up on your life. Don't become complacent.
Or as Paul Tudor Jones once said:
"Don’t have an ego. Always question yourself and your ability. Don’t ever feel that you are very good. The second you do, you are dead."
Related articles and posts:
1. Lessons from Paul Tudor Jones - First Adopter.
2. Paul Tudor Jones: "Trader" - Finance Trends.