Poetry problem (can you explain it? )

This is an ancient poem, so there is no standard requirement for the number of words in each sentence of the poem. In poetry, the number of words generally depends on the poet's emotional strength, and this poem is no exception. The staggered use of five words and seven words in this poem actually echoes the poet's emotional ups and downs and greatly enhances the appeal of the poem, regardless of the strength of the piano.