I see it differently.

Some people are here to serve themselves. (STS)

Some people are here to serve others. (STO)

I learned this dichotomy a few years ago while learning about something that I now think is false; that being, Nibiru.

However, the dichotomy is real. While learning about the above, I decided that I was part of the STO faction. Many of my actions are done because they are the right thing to do -- rather than that they benefit me. In fact, I tend not to maximize the potential benefit to me in my interactions with others -- which is one of the reasons my wife decided to take my ring.