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 Spy, who wasn't sent by anyone to complete any task, is by the side of 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 explanation to things we really need to know about that are essential to the story, like what made event A cause event B. This then leads to Plot Holes turning the story into Swiss 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.

Contrast Plot Said So when a formulaic event takes place. S, The Best Reason is whenever the motive for a character is unexplained or hard to identify.