S, The Best Reason

This is why you can't spell "best" or "reason" without S.

When no other reasons for why a character does what they do are visible, always turn to S. This letter alone can explain in an instant why Villain wants to destroy Hero's town or why the Bad Spy, who wasn't sent by anyone, is by the side of Hero and Co. although the Spy gets nothing out of it.

Contrast Plot Said So when a plot hole goes uncovered or when a formulaic event takes place. S is whenever the motive for a character is unexplained or hard to identify.