Why is The Giving Tree considered controversial?

Shel Silverstein’s book The Giving Tree (1964) has been criticized by some readers for the nature of the relationship between its two protagonists, a tree and the boy she loves. In the story the tree gives all of her apples, branches, and trunk to the boy until she is reduced to a stump. Some people view the story as a metaphor for the unconditional love of parenting, and some have called the story’s relationship unhealthy. However, Silverstein rejected moralistic interpretations of his works and often gave his books ambiguous endings, allowing readers (especially children) to come to their own conclusions and draw their own wisdom from the story.