Aging teaches that energy should be spent more selectively.

You start with the illusion of unlimited energy and spend it accordingly: on arguments that do not matter, obligations you did not want, people who take without returning. Later the supply is visibly finite, and the accounting gets much stricter without any moral improvement being involved.

You do not have to wait for the shortage to spend better. Ask where your energy actually went last week and what it purchased. The obvious waste is usually visible immediately, and removing one item is enough to notice the difference.