Did anyone see the experiment that Derren Brown did re horoscope charts? He found out the star sign of each "subject" and then produced a detailed horoscope for each of them. They were all really impressed that its contents exactly reflected their personalities. When he asked them to exchange their horoscope with the person next to them, they found it was exactly the same as their own! What he had done was include feelings and behaviours that most people could identify with - " you feel nervous going to a party on your own" - or would like to identify with - "you are loyal and trustworthy". Negative traits featured less prominently, as most people are more resistant to identifying with them.
I also think that if you are told that your star sign means that you are "fun loving, spontaneous, ambitious", etc. etc. and you have faith in horoscopes, then you are likely to internalise that "role" and behave accordingly. It's similar to what happens when children are given particular labels - hardworking/intelligent, lazy/slow, etc. etc.
I'm a bit of a cynic about horoscopes and the like but, having said that, I'm also aware that there is no "rational" explanation for lots of things - the philosophy thread's question "why are we here?" for instance.