why did lena ask milkman to come to the tree?

Lena has has enough, she Milkman that he’s peed on everything in the house forcing him to look at the dying maple tree outside. She wants him to remember the day she took him to the woods and he peed all over her.......... ruining her dress and freshly picked flowers. The maple had grown from the twig she took that day.......... it symbolized the way Milkman had never done a thing for anyone in his entire life, and yet everyone had sacrificed for him. And how did he say thank you........... he just pissed on everyone. Lena wants him to know that what he's done has ruined their sister's life.