•Why it earns a slot
A Shropshire Lad earns its place because its spare, classical lyrics crystallized a distinctly modern English pessimism about mortality and lost youth, became widely read consolation during the First World War, and produced individual poems such as 'To an Athlete Dying Young' and 'Into my heart an air that kills' that remain among the most quoted short poems in the English language.
A Shropshire Lad is a sequence of 63 short poems set against the rural English county of Shropshire, voiced largely by a young man who broods on the brevity of youth, the deaths of friends and soldiers, unrequited love, and the consolation of the natural landscape. The poems move from pastoral celebration through elegy and dark irony to a stoic acceptance that suffering is the common lot of humanity and that death is the only lasting rest. Housman closes with the poet-speaker defending his melancholy verse as a kind of inoculation against life's inevitable griefs, invoking the legend of Mithridates who made himself immune to poison by taking it in small doses.
This distillation is written from the freely available original, which is always the better read when you have the time: gutenberg.org.