“Opportunity” in this phrase, found frequently in the commentarial literature, refers to the leisure and freedom to study and practice the Dharma characteristic of a human lifetime free of the eight inopportune states (q.v.); “so hard to find” is a reminder of the rarity of such a lifetime, seen both from a numerical comparison to other forms of existence and from the improbability that an individual might gather together the meritorious causes and conditions that give rise to it.