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 the Villain wants to destroy the Hero's town or why the Spy, who wasn't sent by anyone to complete any task, is by the side of the Hero and Co. although the Spy gets nothing out of faking a good relationship with the heroes.

When S is taken too far, we're left with too little or too bad of an explanation to things we really need to know about that are es'sential to the story, like what made event A cause event B. This then leads to Plot Holes turning the story into Swis's cheese. Finally, we're left with a Confusing Story that doesn't give anyone any answers to questions they have. And it's all because the creators thought S could explain everything for them.

Contrast Plot Said So when a formulaic event takes place in the story and we don't need to ask why it happened. S is whenever the motive for a character is completely unexplained or hard to identify.