This poem is by Ernest Dowson (1867-1900). Merely discussing him is a sad matter, because Dowson was both a student at Oxford for a time and a severe alcoholic whose life ended far too early. We can extend the parallel further in he was a Roman Catholic by conversion.
We should not be surprised that he titled his poem in Latin; this was in the days, after all, when a knowledge of Latin was considered indispensable to a good education. So that is why students of English poetry find themselves faced with these Latin words at the head of the poem:
Vitae Summa Brevis Spem Nos Vetat Incohare Longam
It means, essentially, that the brief (brevis) sum (summa) of life (vitae) forbids/prevents (vetat) us (nos) beginning (incohare) a long (longam) hope (spem). But we can think of it as meaning simply:
The Shortness of Life Forbids Us Long Hopes
The phrase comes from lines in Ode 1.4, by the Roman poet Horace (65-8 b.c.e.):
Dowson is speaking of the brevity of human emotions. Weeping and laughter, love and desire and hate, he says, do not last long, and he thinks they end with death (“passing the gate”).
In like manner, he tells us, the days of pleasure and happiness, which he poetically terms “the days of wine and roses,” are not long either. And as for our short life, it is like a path seen coming out of a mist, then disappearing into that same mist.
Dowson’s poem is undeniably beautiful. Happiness is brief, life is short and vague and a mystery, but in reading those lines by Dowson we must say that, as R. H. Blyth once remarked, put that way, it doesn’t sound too bad.
Dowson did have a sense for the poetic phrase. Many who have never read his poem know the words “the days of wine and roses,” which were used for the title of a movie about a descent into alcoholism. And it is from another poem by Dowson