The classic explanation for why we feel astrology has a degree of accuracy is that we all possess the full range
of human characteristics in varying degrees, so any sufficiently vague sign description can seem
to make sense to us. And horoscopes are usually even more vague than the sign descriptions -- "Be wary of those who would like to see you fail today," "Use your best judgment when taking financial risks this week." Sheesh!
Here's the thing that sets us up, I think: Astrology is so pervasive that most people are exposed to it at a very young age, and therefore it does have a powerful influence over us -- but an influence of a perfectly terrestrial
kind. I submit two youth-based theories:
(1) We're so young when we learn about astrology that we don't yet have the intellectual capacity to approach it with the healthy skepticism we should apply to any unproven belief system. Therefore, we tend to initially accept it -- and by the time we realize how suspect it really is, we've already had enough "gosh that sure sounds accurate"
experiences (see "classic explanation" above) that it seems
to have shown some validity... so we don't shake off our initial belief as easily as we probably should.
(2) We learn very early on about the characteristics traditionally associated with our "signs" -- and our very awareness that we supposedly possess those qualities causes them to become emphasized in us as we develop. We take some sort of odd satisfaction and comfort in living up to the description of the stubborn Taurus or the hot-tempered Scorpio... and astrology thereby becomes a self-fulfilling prophecy.
That's what I think, anyway... and I think a lot,
because I'm an Aquarius.