You attempt to test the hypothesis that the problem you're trying to solve resonates strongly with the currently unmet needs of a beachead market.
Since these customers will resonate strongly with you, they will not only pay you from day 1 but advocate/market/refer for you.
The hard work is in testing this hypothesis first instead of falling in love with the project ideas that you would like to work upon, and caving into building them because you would really like to work upon them.
But: I think the real motivator is if the idea solves a problem the founder is personally experiencing.
If there were at least one reproducable answer, we'd live in a completely different world.