In this song tree is metaphor for human being. Same as the tree, we too, grow up, spend some time on the Earth and then slowly die.

We can see that in early days he was full of life, filled with joy and happiness, but as years went by and he got older, dark clouds appeared on the horizon. He realized that his best days are behind him and this caused sadness in him.

Song does great job at expressing sadness and depression in person. From lyrics, to pain in his voice, it greatly shows, how upset the person is over the fact, that he will never be young, strong and healthy again and that as every day passes, he is one day closer to inevitable death.