Well, when you're in the teens, the chemistry is still working strongly in various directions. It's like trying on outfits to see which one is the best suit for you.

As you mature, new values become more prominent. The chemistry is a little more calm permitting you to see things from a different perspective and your appreciation for healthful behavior is more important.

People have 3 basic drives: the drive for reproduction, the drive for power, and the drive for social approval. I would say that your appreciation is related probably to the drive for social approval--the feeling-well tone of knowing that you look good, are healthy, and feeling love for yourself and so are able to love others.

When the drives are all in balance, things are at their peak of performance and
permit a very healthy, happy lifestyle, barring no mental illness.

Good wishes.