Bearwalk9, the answer really depends upon whom you ask.
Back when horoscopic astrology got started, the Hellenistic astrologers took a neo-platonic view. The envisioned a divine consciousness that permeated everything (translated as "God" though not in the strictly Judeo-Christian sense.) They reasoned that because the human body encapsulated part of this divine mind, that humans were capable of understanding astrology as part of a divine plan. Their gods, whether understood literally or metaphorically, were part of this divine plan, as were the planets. This was a holistic system. Moreover, the gods might change their plans through human prayer or sacrifice.
The gods that gave the planets their names and attributes were human creations: they looked, spoke, and behaved like human beings. Mythology was the great literature of Antiquity-- and some would say, of today. Consequently the myths can be interpreted as allegory and metaphor for the different types of human experience. Venus, for example, is not just a Roman love-goddess, but symbolizes human love with all of its shadings of meaning. Thus the myths of the goddess Venus show her as amorous towards lovers, affectionate towards her child, jealous of her rivals, exemplifying feminine beauty, and so on.
For people today of a religious or literary frame of mind, this still makes some sense. Great literature reveals truths about the human condition, even though we know that a novel is a work of fiction.
I don't think mechanistic cause-and-effect explanations such as gravity or electro-magnetism really work. The interactions between earth and planets beyond the sun and moon are negligible. The physics explanations don't explain the level of detail we see in astrology: why, for example, a woman with Venus in Scorpio square Saturn might be prone to jealousy. Even if we imagine that the planet Mars, say, affects iron in the blood, it doesn't explain why Mars should be domiciled in Aries.
Moreover, cause-and-effect explanations borrowed from physics do not explain horary astrology at all. Why should a horoscope for a moment of a question explain where you left your missing car keys?
Ultimately the question of "why astrology works" (assuming it does) remains unanswered.