[prirun]

Fingering is largely a planning problem. If your RH thumb is on C and the next note is D, it's obvious you play it with 2. But if the next note is the B below, you'd have to do something awkward to play it. So having your RH thumb on C is wrong and you need 2 on that to play the B with your thumb. Unless the next note is the A below. :-)

If you look ahead a bit at a line of music, you can sort of anticipate how many fingers you'll need in the direction you're going and that helps plan which ones to use. But as others have said, knowing how to play scales contributes a lot to your planning skills.