[nobody0]

I'd say always start with what. Figuring out what to solve is the most vital part.

[once_inc]

In my experience, that often leads to solving issues for nonexistent clients, or bad product-market fit.

[chasd00]

I think the “what” has to be followed by a “because” or, like you alluded to, you end up solving a problem no one cares about.