In theosophical literature, Helena P. Blavatsky affirms that the Dweller is not a fictitious entity. It is the discarded astral double of an individual in a previous life that may not have fully disintegrated yet when that individual is reborn. Thus the dweller will be drawn to the new incarnated personality due to their affinity. Sometimes this entity is also called Guardian of the Threshold.
Another explanation is that provided by Alice Bailey on Esoteric astrology, "From ancient recesses of the memory, from a deeply rooted past, which is definitely recalled, and from the racial and the individual subconscious (or founded and established thought reservoirs and desires, inherited and inherent) there emerges from individual past lives and experience, that which is the sum total of all instinctual tendencies, of all inherited glamours, and of all phases of wrong mental attitudes; to these, (as they constitute a blended whole) we give the name of the Dweller on the Threshold."
This Dweller is the sum total of all the personality characteristics which have remained unconquered and unsubtle, and which must be finally overcome before initiation can be taken. Each life sees some progress made; some personality defects straightened out, and some real advance effected. But the unconquered residue, and the ancient liabilities are numerous, and excessively potent, and - when the soul contact's adequately established - there eventuates a life wherein the highly developed and powerful personality becomes, in itself, the Dweller on the Threshold. Then the Angel of the Presence and the Dweller stand face to face, and something must then be done....(Occult Magick)