Which line from the poem "loveliest of trees the cherry now" best support the theme that people should take pleasure in nature during their short lives?

In the following lines from the poem "loveliest of trees" by Alfred Edward Housman it is best supported the stated theme:

"Now, of my threescore years and ten,

Twenty will not come again,

And take from seventy springs a score,

It only leaves me fifty more."

The author makes reference that life is short as he is in his twenties and he only has just 50 years more to enjoy it (50 spring to admire the beauty of life).