Consider the story told by the poem as a whole.
In the first stanza, "young girls" take all the flowers, picking them for bouquets, wreaths for their hair - no reason is given, but the flowers are gone, "every one." The refrain at this time seems to be asking when the girls will learn to leave some of the flowers to continue growing, to be beautiful for someone else.
In the second and third stanzas, the young girls have gone to take husbands, and the young men have gone to become soldiers. The refrain may be questioning the wisdom of making commitments and getting married in unsettled times, or may be questioning the wisdom of requiring young men to become soldiers. Remember, this poem was written in 1961, as Vietnam was becoming a concern for the United States.
In the fourth stanza, the soldiers have "gone to graveyards every one." When will people learn to solve their problems without warfare and killing and destruction of life, the refrain asks.
The final stanza brings the story full circle, back to the starting point. The flowers that were all being picked at the beginning of the song are now covering the graves of the soldiers, presumably placed there by the "young girls" who are now young widows. Again, the question is when will we all learn that there must be a better way to resolve conflicts than sending young men to war.