In "Loveliest of Trees, the Cherry Now," why is the poet lamenting?

In Housman's poem, a young man of twenty enjoys the snowy white beauty of the cherry bough in bloom in spring. Yet at the same time, he laments how brief life is. Even if he lives to seventy, which he considers his full life span, he will only have fifty years left to see the cherry bough in bloom.
Fifty years might seem a long time until we remember that the cherry tree blossoms in all its glory only for a brief time each year. The ephemeral or short-lived quality of the cherry's blooms represents or becomes a metaphor for how brief life itself is. Blink your eyes and it is over. Life is beautiful but it speeds by too fast. The poet is sad about this, but his acute consciousness of life's brevity means he will make time to go to the woodlands to see the cherry tree. He understands he needs to make the most of the time he has, and the poem prods the rest us to be just as alert to how short life is, so that we too can fully enjoy its beauties.