To me proof-of-work is also like excercise / body building.

You work hard for an hour, look in the mirror and there's nothing to show for it but you trust the process. Week after week you toil away, still nothing... but eventually something changes, something looks a little different from that first time you looked in the mirror. A year or two later you see a real difference. Did you get lucky with genetics? No, you put the work in.

You can trust the process because you're the one slaving away each day and you can verify it by what you see in the mirror. This is your own personal proof of work.