People should do what lights them up inside, because you'll be passionate about…

People should do what lights them up inside, because you'll be passionate about it, you'll think about it all the time, you'll think about it while you're in the shower, you'll think about it while you're driving, and you might even think about it enough to do something remarkable because of that passion.
If you're just doing something just because you think it's important, even if that something is morally good or right, your efforts will end up second rate, just fulfilling that importance or duty. Your efforts will not be true to your heart, your passions, or yourself.

TLDR: Follow your passions if you want to be happy in life