I love this Derek Sivers piece about finding your work in the sweet intersection of these three factors. Really enjoyable breakdown of what you get when you’ve only got two of them and have forgotten to account for the third. He concludes:

When a life or plan feels ultimately unsatisfying, I find it’s because I’ve forgotten to include:

• what makes me happy
• what’s smart (long-term good for me)
• what’s useful to others

Related:
Work
Struggle
Happiness