The souls of Elf-kind are a finite thing. When Loran dreamed them into the world, she knew each of them by name, she knew their heart and the very fabric of their essence that made them unique. As such, there are a limited supply of souls for the elves to possess.
Usually, a soul will be joined with an elf at birth having no recollection of its past journeys through the cycle. Though a place may seem familiar or perhaps even a person, there is really no way of knowing the past experiences of one's soul. The experiences made through the journey of a life are all unique to that existence, holding no responsibility for a souls past actions.
When an elf dies, their soul is placed back in the great wheel for Brundir to help along the celestial path, eventually back in to a new life.
If a soul is reaped, it is plucked from the wheel completely and never replaced. This is what makes Estril's burden so great. The finality of it is felt in ripples, and the loss is deep and eternal.
It is rumored that such a thing as an Awakened Soul does exist, though it is never spoken of openly. An elf remembering their past lives is thought to be broken and flawed, unnatural and likely corrupted.