There are two key factors at play - temperament and reinforcement. 

Parents may over-emphasize temperament and over-value personality.  We do it all of the time.  "That's just his personality."  Strong-willed individuals may require more work.  Defiance may require more work.  Thus, shaping and reinforcement for that child's individual needs requires something different.  

See Dr Dan’s new book: https://www.amazon.com/dp/1935576062/?tag=medrounpublic-20