Theory:

The entire poem is centred around a fight that Eleanor had with her brother. It shows how fights start and end between siblings. The perfect emotional train of denial, anger and acceptance is shown in the lines.
 
1. Denial: The first takes us directly to the fight. She admits that she doesn't know the reason they had fought, showing that the reason was not important and the emotions played a bigger role in the fight. The reason would be tiny and in most cases, not worth fighting. She says that the beginning of the fight was little, but it soon grew with time. She ends the first stanza by telling us that both of them thought they were correct with their ideas and were not able to accept the other person's idea.
 
2. Anger: Although they were siblings and had grown up together, this fight between them had caused them to hate each other openly. Time passes quickly, and the afternoon becomes evening while both of them had continued hating each other. The anger that one sibling has over the other is expressed in two short yet beautiful verses.
 
3. Acceptance: Soon, Eleanor's brother takes the first step towards solving this fight by playfully thumping her on the back. It shows that no matter how much they fight with each other, the reality is that they can never stay angry forever. The poet realises that by telling her that he was wrong, he was actually correct, so when he accepts his fault, she accepts her fault as well, and the fight ends. It shows how when two members of the same family fight against each other they can never stay angry forever because they know each other very well and the love that they share is far stronger than any fight.