One way to break down a lifetime would be to think of it as two portions-- the part where the person became better, and the part where he coasted.
In a normal person's life, the getting better part would include everything from his first breath of air, as he learned how to see and feel and breathe, through school as he learned different things, and probably through the beginning part of his job as he developed a baseline proficiency in his trade. The coasting part would be most of his career, as he put his educational investment to work, and, of course, retirement.
There are a lot of ways to get better. You can learn new things. You can travel and see the world, thus gaining new perspective. You can build your personality. You can create a body of meaningful work. You can become more healthy and more fit. You can actively cultivate relationships with people.
At some point, most peolpe will all but stop these things. I've noticed two big catalysts. The first is when someone gets a job. The job validates that they have skills, whereas school tells us that we need skills. Most people in any given company aren't improving themselves anymore, so by taking a job you become surrounded by a peer group that's no longer focused on bettering themselves.
The other big catalyst is marriage. Once a person is "chosen for life" by another person, they get out of shape, pick up their partner's negative characteristics (because it's easy to slack, especially when it's accepted), and stop developing their personality.
These two things don't HAVE to stop people from getting better, but without actively managing their paths in life, most people succumb to it. Actually, a marriage between two motivated and self-aware people SHOULD be a good opportunity for mutual self improvement.
I tend to take extreme positions on certain issues, and this is one of them. I think that when you stop getting better, you're essentially dead. The human experience is one of growth and pushing limits, and once you stop doing that, you're done.
When you're constantly becoming better, you get to see the same thing a few years apart and have it mean something different the second time. You understand it from a different perspective and learn something new when you see it again. It's the foundation of a rich life.
You also have more to teach other people. Think of the stereotypical old guy who tells the same stories over and over because he doesn't have any new ones. Most people are already like that by the age of thirty.
Over the years I've probably called a dozen different things "the most important thing you can do". Let's make it a baker's dozen and add this one to the list. If you have an attitude of constant curiousity and self improvement, I think you get the most out of life possible and give the most back possible. What's more important than that?
As penance for slacking on posting lately (and just because it's a good thing to do), I'll be posting twice a week for at least four weeks.