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I think I have a decent answer to the question "what is the most important thing in life". It is to deliver the maximum amount of utility to other people. This is the direct opposite of accruing the maximum amount of utility for yourself. You provide utility to people by delivering value. Everyone has a utility function of how important a certain amount of value to them. If total utility is T and a utility function is U and value is v, then T = U(v). Utility and value are generally positively correlated (I am usually happier if I have more money or friends).

Value is not just financial (a common fallacy), but can also be emotional. Having a loving family is very high utility to you, but you are also providing high utility to your family members by supporting them. This could explain why billionaire entrepreneurs may be unhappy with life if they don't have any close friends or family, because they are very poor (not rich) emotionally. If the most important thing in life is to maximize utility for others, then being the sole rich person yourself is equivalent to zero wealth if you haven't delivered any utility to others. Nobody is happy about Trump making the White House fancier when they cannot afford food (and he is suing for an emergency stop on issuing food stamps).

It is important to note two aspects of delivering value. First, people generally have logarithmic utility functions. If you deliver all the value to a single person and zero value to others, that is not very high utility. Actually, this really means that you want to maximize the sum of utility you have provided other people. In theory, providing ten poor people $100,000 per year generates a much higher overall utility than providing one poor person $1,000,000 per year. Likewise, providing extremely high emotional support for one person and neglecting everyone else generates less value than providing high emotional support for multiple people (such as spouse+kids+family+friends).

The general (all types summed) utility function is logarithmic, but the constituent financial utility and emotional utility functions are also logarithmic. This is captured in the inwardly-bowed preference curve in economics. Poor people with lots of friends value money more than rich people with no friends, and the rich people with no friends value friendship more than poor people with lots of friends.

Also, people's utility functions change over time. This is why it makes sense that providing financial stability for your parents when they are 70 years old benefits them more than waiting until they are 80 years old to provide them immense wealth. The utility of providing them value earlier is higher; utility functions are not constant over time.

Therefore, there's three optimizations happening simultaneously:

1. How do I maximize the amount of utility I provide for others over the course of my entire lifetime?

2. How do I allocate value across people such that the allocations maximize the sum of everyone's utility, subject to fairness?

3. How do I allocate value across time such that the allocations maximize the sum of everyone's utility, subject to fairness?

This boils down to "be good and help everyone especially when they need it", since we are optimizing across self, relationships, and time. These three things by themselves are very important to life, so this seems to be a good smell check that this idea is ballpark correct.

Finally, it is important to remember that these are off-the-cuff thoughts by me at 21 years old. I will likely mature and have more nuanced or accurate thoughts about what is the most important thing in life when I get older. Also, life is very complex and it's impossible to completely distill it into simple theorems or ideas.

I'd really appreciate hearing other people's thoughts.