Ammon's relationship with his father has affected his entire life. Ignored, forgotten, considered irrelevant, his father made sure Ammon knew how little he thought of him. Even though he now has his life together, Ammon still can't let go of his animosity towards his father, which is tough to resolve considering his father died last week.
Ammon's brother Benjamin, on the other hand, was the favourite. Loved by his father, Benjamin's biggest struggle comes in letting go of the man who loved him so much. Hence why Benjamin stole the hearse from the funeral, hence why Ammon jumped in to get him to stop, hence why they're both sat on the side of a road with their father's body in a coffin behind them.
As they talk they both begin to understand each other's relationship with their father through a different lens.