Two major explanations exist. This methaphor might refer to the painfulness of love: the smoke, when it gets to the eye, is painful, it even make someone cry, such as love – which is, despite of being beautiful at the same time, makes people ofthen suffer.
Another explanation is, that where there is smoke, there is also fire – in the lovers eye. Somke has to be fed by something – so love itself is only something secondary. But what lies deep inside is more important: it is real fire, real pain which makes you work and live or suffer either.