Exactly. It's not that John is deliberately ignoring Sam —he's in pain and it's a bad time of year and Sam is an argumentative little thing on a regular basis. Sure, it doesn't make him father of the year, but his reaction is understandable at least. And of course Sam is fourteen and his response to a perceived rejection by his father is to feel put-upon and sulk, because that's what most fourteen-year-olds do rather than try to walk a mile in someone else's shoes. In short, it's a recipe for disaster.

My own parents, who are right at the top of the list of Awesomest Parents Ever™ made a couple of blunders like that in their time, because all parents do, at some point or another. Doesn't mean they don't love their kids or that they're inherently bad parents. They're just human.