In the eighth season of the show, the issue of why Ned very seldom ever shows his anger was greatly explored in the episode " Hurricane Neddy ". Archived from the original on Homer is the last person that Ned comes to and when he does, instead of shouting at Homer like how he did to everyone else, Ned just in a calm voice tells Homer that he's the worst person that he has ever met.
A Streetcar Named Marge
However, one of his relatives; Lord Thistlewick Flanders, does not have this trait other than when pressured by Ned Flanders, which may imply that his relatives might just attach the trait when with Flanders. Richmond, Ray ; Coffman, Antonia eds.