To be honest, there's no causal link between the relative positions of the stars as viewed from wherever Earth happens to be and the developed mental states of human individuals.
In other words, it's a crock of shit. But it's a nice crock, with beautiful culture and art, a rich history, and an alchemist-like code of conduct. If you accept that it is no more than random opportunities to guess someone's personality based upon arbitrary planetary positions and archaic celestial mechanics, then you can enjoy it even more.
You should check out the horoscopes over at
The Onion.