At the end of the article, the tree said: "The tree is very pleased, but the bottom of my heart is even more sad." Talk about what is the "happiness" and "sadness" of trees.

I want to compare the trees here to our parents, who devote themselves wholeheartedly and expect nothing in return. If they have their own lives, they will gradually move away from their parents. If I explain it this way, the tree is gratified by what they can do for their children when they are old. The bottom of my heart is even sadder because even if their children are old, idle and have more time, they will not spend more time with themselves.