The , 'to become', like any infinitive, can be used the way a gerund is used in English, so that, for example, means 'the boy's going didn't please me', and can mean 'his/her/its becoming dust'. But as Nazm points out, the second line, while grammatical, sounds somewhat awkward. I've tried to keep both the literalness and the awkwardness; otherwise, the normal English form of the question would be 'why does it become dust?'. The verse has a larger problem, though. Its parts just don't assemble themselves into an exciting or revelatory whole. The attributing of desire or lust [] to something as impalpable as rose-scent is justified by a set of three inert constructions: the rose-scent becomes 'dust of road of movement of breeze'. In other words, it places itself humbly and free-floatingly, like dust, in the path of the breeze, hoping to be borne toward the beloved's street. Thus the cloud of rose-scent is, somewhat awkwardly, depicted as feeling a strong human emotion, but acting in a feeble, vague, and passive way to express it. What (if anything) energizes the verse is the lover's pathological jealousy. He is morbidly suspicious, and always on the lookout for a Rival , even in the most improbable places. Thus his interrogative approach, and possibly a hectoring tone to go with it: 'Oh, so you've been flirting again-- I knew it! Hah! That's why the rose-scent is pursuing you, isn't it?' The argument could also be made that the verse implicitly plays with the double meaning of the (Persian) word , which can mean both 'desire' and 'wind, air'. If so, it's a subtle form of play, since the word doesn't appear in the verse, unless we consider the (Arabic) word to be a sort of evocation of it. For an explicit play of this kind on , see 48,10 . graphics/dust.jpg