There is, as yet, no satisfactory scientific explanation why Venus or any other planet should do this. But then, love itself is not a scientifically provable phenomenon. To experience it, you have to stop asking what it is and just give yourself over to something you feel to be true. If you're going to say that Venus is only a planet, you may as well say that a rainbow is nothing more than an intriguing phenomenon involving the refraction of light. Or that a kiss is only an expression of some primal urge to reproduce the species.
Romance defies logical explanation - and so, to some degree, does astrology.
But that doesn't make either of them any less real.