Feelings change with the change of things, and feelings are produced. Things that used to be happy get old in a blink of an eye, but they still have to arouse feelings in their hearts. Moreover, the length of life is left to nature and finally comes down to elimination! The ancients said, "Life and death are important things." How can you not be sad?
The author's feelings changed from joy to sorrow, not only because joy begets sorrow, but also because he thinks that he is getting older and his life is limited, but he has not yet realized his ambition, so he can only leave his feelings in the mountains and rivers and indulge in debauchery. How can he not be sad?