As we see more and more these days, more and more young adults are going to college and/or some kind of schooling/training after high school, much more than people did during my parents' generation. There's numerous reasons for this, but the main reason I'm hitting on today is continued growth. People are realizing that in this competitive world, many of us are obsolete if we stop growing after high school. This isn't to say there's anything wrong with not wanting to continue your education after high school, but no matter what the field of profession, you have to at the very least do some kind of training that wasn't received in high school or before in order to succeed at said profession.
To tie this back into the martial arts world, once that black belt rank is achieved, it doesn't stop there if you want to succeed as a martial artist. My original PaSaRyu instructor once told me that "earning a black belt doesn't mean you've gotten there; it means you've earned the right to get there." I'm not necessarily saying that you need to train until you a 10th dan, but realize that your growth hasn't ended simply because that cloth around you waist is now black.
One of the best ways to continue growth is to teach others. Anyone who has ever taught anything in life will agree with that statement. When you have to teach someone else something that you've learned, it makes you apply what you've learned so you're no longer just going through the motions.
One of the things I love about the PaSaRyu style that is unusual to other styles and schools I've been to is that it is very common (almost required in some PaSaRyu schools) for colored belts to teach lower colored belts. In addition, there's an unwritten rule that the highest ranking person in class--even if that person happens to be a blue belt--runs the class (granted I've only witnessed this maybe twice in my 9 years of training, and even then it was only because the chief instructor was running late). In fact, we actually have an Instructor ranking (technically, it's called "Brown Belt Instructor") that falls between brown belt and red belt. This may cause those that are accustomed to black belts doing all the teaching cringe at the very thought of this, but it does help the colored belts reinforce what they've learned, and as a result are much better instructors once they become black belts themselves. They also learn something I learned from my instructor they very next class after I myself earned the Instructor rank, what I call "The Golden Rule for Instrutcors,"--put the needs of others before you own. This ties into the traditional Golden Rule, in addition to treating others as you want to be treated, teach others as you want to be taught.