How does the poem's rhyme scheme help to advance its meaning?
The simple rhymed couplets chosen as a rhyme pattern give the poem a child-like, simplistic tone, almost like a nursery song (“itsy bitsy spider,” etc.) that renders the naivete, the innocence that the poet wants to give the lament. Youthful exuberance, accomplishment, fame, promise – before age, maturity, irony, loss of innocence takes the childlike joy away – while on the surface the poem celebrates youth, it is really about the loss of that youth, the loss of naivete. The rhyme scheme, simple by literary standards (compare, for example, the sonnet forms); Housman was not unfamiliar with blank verse and other modern trends, so he chose this rhyme scheme to imitate that loss of innocence that modern poetic form itself had lost. In addition, the rhyming words are all monosyllabic, and the poetic feet are all iambs, and each line has a masculine ending (i.e., stress on the last syllable.) All this speaks to simpe, naïve, youthful accomplishments which would have been tainted by experience, had the athlete (read poet) died young. At the extreme of interpretation, we could say that Housman felt he had outlived his best poetry.