I don't know what's better gettin' laid or gettin' paid.
Flow is being completely involved in an activity for its own sake. The ego falls away. Time flies. Every action, movement, and thought follows inevitably from the previous one, like playing jazz. Your whole being is involved, and you're using your skills to the utmost.
You should be doing something that you're passionate about and that you think is really important.
The hardest part of any challenge is taking the first step.