What is the ‘sadness’ that the poet refers to in the poem?

Sadness is the condition or quality of being sad- unhappy- sorrowful- depress and low spirits which is formed out of Man's own thinking and action. A human is the most intelligent animal on this planet yet he fails to understand himself and his actions. His Rush and Hurry for the worldly glories has cause horrendous trouble and problems for not only himself but for nature too. Man is the author of all disasters.