For everything, there is a reason.
--Not in life, necessarily--but, when writing fiction, there had better be a clear rationale for why your character said what he said or did what she did.
Actions and reactions should be linked from page one all the way to three hundred and one. Each scene and/or event should logically progress from one to the next.
Dig deep, and know your character's "why" (even if he or she doesn't know it at the time.)
In novels, things don't happen "just because."
There has to be a reason.