Interesting point. If we really weighted our own wellbeing exactly the same as the wellbeing of others, we would just put our energy wherever it would be most helpful, regardless of whom we were helping. But we're not psychologically built to really divide our caring by 7 billion people. Anyone who tried to divide their energy among that many would probably give up or die. People like Buddhist monks who put a lot of practice in may achieve this on some kind of psychological basis, but I don't know of anyone who actually acts on it all the time.

In helping professions (nursing, social work, etc.) you're taught to take care of yourself so you don't burn out. Which does mean putting your own wellbeing ahead of an individual client's, but in the long run it allows you to give more people better service. I think this is good practice for philanthropists, too.