It's in our own self-interest to serve something other than ourselves.
Serving yourself is only successful in the short term. Eventually, it leads to all the diseases of nihilism – compulsive acts of consumption. The person on the street drinking mouthwash and the hedge fund manager who just bought a yacht have more in common than both of them think. In a second, it can all be gone.
It's better to serve something else. But choosing can be terribly difficult.
Some lucky people stumble into situations where what to serve is clear. Some activity they enjoy, fulfills them socially, provides a sense of purpose. People who dedicate their lives to music at an early age – Artists. Deeply religious folks. Surfers, ski bums, perpetual travelers.
Some get it from their job. Social workers, Health care workers. Soldiers, Scientists. Barbers and bartenders.
But if you don't get everything you need from one bundled source, then you have to balance competing interests. You gotta choose.
Choosing is hard because of opportunity cost. An infinite number of things we must choose not to do every time.
If you have an activity with no correct, deterministic answer, but which instead requires choosing between tradeoffs, it's an art, not a science. Most of life is an art: it has limitations, and requires practice and creativity.