“In real life, coincidence happens all the time. But in fiction—especially when the coincidence helps the character be at the right place at the right time, or overhear the crucial telephone conversation . . . coincidence is deadly.”The same principle applies to motivation. People don’t do things without some goal or provocation in mind, and when they do behave that way in fiction it is less plausible, and therefore less satisfying. Background is one aspect of motivation, and needs to be portrayed logically.
“A great many stories tend to be unbelievable because the writer just shoved a character onstage to do something without thinking through how and why the character got there.”
#10 Don't Have Things Happen For No Reason