I think that this book was written to show how far a parent would go for their child. When I read it though, I leave out the last couple pages, because I use this story a lot to show the impact of codependence on anything (substances for my treatment). The tree represents the patient, and the boy represents drugs. Sometimes I pull in some RFT principles and have the patients imagine what the experience is like for the apples, or the leaves, or the boat.
Plot from Wikipedia
The book follows the lives of a female apple tree and a boy, who develop a relationship with one another. The tree is very "giving" and the boy evolves into a "taking" teenager, man, then elderly man. Despite the fact that the boy ages in the story, the tree addresses the boy as "Boy" his entire life.
In his childhood, the boy enjoys playing with the tree, climbing her trunk, swinging from her branches, carving "Me + T (Tree)" into the bark, and eating her apples. However, as the boy grows older, he spends less time with the tree and tends to visit her only when he wants material items at various stages of his life, or not coming to the tree alone (such as bringing a lady friend to the tree and carving "Me +Y.L." (her initials) into the tree. In an effort to make the boy happy at each of these stages, the tree gives him parts of herself, which he can transform into material items, such as money (from her apples), a house (from her branches), and a boat (from her trunk). With every stage of giving, "the Tree was happy".
In the final pages, both the tree and the boy feel the sting of their respective "giving" and "taking" nature. When only a stump remains for the tree (including the carving "Me + T"), she is not happy, at least at that moment. The boy does return as a tired elderly man to meet the tree once more. She tells him she is sad because she cannot provide him shade, apples, or any materials like in the past. He ignores this and states that all he wants is "a quiet place to sit and rest," which the tree, who is weak being just a stump, could provide. With this final stage of giving, "the Tree was happy".
Hexafles processes addressed
Fusion - The Tree fuses to the thought that it can only be happy when The Boy was around or when she doing something for The Boy. It even becomes literally fused to when intitials are carved into The Tree.
Experiential Avoidance - The Tree will do anything she can to avoid making The Boy feel like he can't do anything, because that would make her sad.