Housman, as throughout the poems of "A Shropshire Lad," sees life as fleeting and therefore to be enjoyed when one can do so, before it's too late. The view expressed is that of a young man who already recognizes, at the age of twenty (because in biblical terms a man is supposed to be granted 70 years to live—"three score years and ten") that he only has fifty more springs to view the cherry trees. He must therefore take advantage of all the time that remains, and not miss the beautiful cherry blossoms when they are to be seen.
In spite of a theme that's tinged with melancholy as it is, the tone of the poem is actually much less dark and pessimistic than that of "A Shropshire Lad" as a whole. The speaker is basically joyful, stating that life does have more to offer than pain and regret, a message which one would not gather from Housman's poetry in general.
A significant point, nonetheless, is that the speaker is seeing the cherry boughs in spring "hung with snow" and "wearing white for Eastertide." In other words, the signs of winter have lingered. Even here, when the beauty of nature is the speaker's focus, it's a beauty that is ambiguous in a way, because it is draped in the snow of winter. Housman's message is arguably that the darkness, the cold of what for many people is the saddest season of the year, still haunts us even at Easter, a time of renewal.