One of the most important things to remember as a diver, is to keep learning. Even as an Instructor, I try to learn something new on each dive. It may not be a huge learning experience, but even the smallest thing you pick up makes you a better diver.
Learning keeps us thinking, and thinking keeps us sharp. The one thing you don't want to do is become complacent. By doing so, you tend to "let things slide". One of the fun parts of diving is trying to be the best diver you can. That is accomplished through knowledge. The people I dislike diving with the most are the "know-it-all" divers. They tend to be reckless and unsafe.
To be learning as a diver, does not mean you have to constantly take certifications, (although thye definately can be helpful), but simply pick up on new ideas, or techniques used by others. Diving is the best sport on earth, (OK, I think it is), and we should all strive to be the best diver we can be. This will improve the sport for all of us who participate.