It is estimated that every year, Americans spend hundreds of millions of dollars on horoscopes and astrological forecasts. This proves that no matter how the stars align, there is, in fact, "a sucker born every minute."
Many, if not most, people who read their horoscopes will tell you that they only do so for entertainment purposes and don't actually give any credence to the forecasts. But many of those same people will also tell you that they "know lots of people who really fit their descriptions." You may be familiar with those descriptions yourself--people born under the sign of Taurus are bull-headed, Capricorns are stubborn goats, etc. etc. etc. And lots of people believe they fit their own signs especially well.
How do astrologers achieve such uncanny accuracy? By making the descriptions generic, often contradictory (you enjoy people but also need to be alone sometimes, right?) and by filling them with things people want to believe about themselves. Tell someone that people of their sign are independent thinkers yet they value the insight of others while remaining true to their ideals but very adaptable, and most everyone will go "Yeah! That's me! I'm terrific like that!
"My greatness is written in the stars, just like I always said!"
From there, you need only make up some daily predictions involving sex or money, pulled not from the stars but from Uranus (sorry), and let confirmation bias do the rest of the work. No one will remember all those days you predicted sudden wealth or romance that didn't happen; they'll certainly remember that time you warned of impending catastrophe and they cut off a toe.
"I've been thinking about you for weeks and suddenly you call! We must have a psychic connection!"
The human brain evolved to find patterns in nature and extrapolate them into the future. That is the key to our success as a species. Unfortunately, the brain can never be a perfect instrument for apprehending reality; mistakes will happen. And if there is a way for evolution to select for brains that tend to make less costly mistakes, it will.
There is, and it did. Because a false positive in pattern recognition (seeing a pattern in randomness) is usually less costly than a false negative (failing to see a real pattern), our brains have evolved to err on the side of false positives. In short, it's safer to think a rain dance works than it is to miss the fact that everyone who drank from that pond died the next day.
Or, to put it another way, failing to see the tiger's face hidden in the bushes will kill you. Seeing the face of Jesus on your taco will not. So your brain tries really hard to see patterns in everything, because the penalty for missing the real ones is too stiff.
You have thirty seconds to live.
You have thirty seconds to eat this before it gets cold.
So, of course, when ancient man looked up into the sky at all those stars, it was perfectly natural to connect the dots, seeing archers, fish, guys with buckets, and all manner of fantastic beasts. And then to see patterns in the positions of the sun and moon and planets that correllated with stuff happening around them.
Or, "You're Carrying Your Metaphor a Little Too Far"
Okay, so you've got a bunch of people and animals populating your sky. You look at the stretch of sky through which the sun and planets appear to travel, note which sky critters they appear to be near, and try to find meaning in those juxtapositions.
Obviously, the particular critter in that part of the sky must have some symbolic meaning, right? If the sun was among a certain collection of stars (which you imagine to be, say, a crab) at the time of your birth, you must have crab-like traits, right? So obviously you are the type of person who hides away in a shell, yada yada yada. Gemini, the twins? Right, split personality.
"You have an exoskeleton, and live in an underground burrow, where you will find true love."
This is a version of sympathetic magic, a belief that traits apparent in one object can be magically transferred to another, or to you. It's like believing that eating a whale's gigantic dick will cure your impotence. Unfortunately, science has proven that this sort of belief will only result in being asked to plug in the vibrator on your way out of the room.
Not only that, it will taste like whale dick.
Astrologers continue to fail to account for precession of the equinoxes, a "wobble" in the earth's rotation that affects the apparent position of the sun relative to the stars. The sun appears in a different part of the sky for any given date than it did when astrologers first laid out the zodiac and determined the "signs" still in use today.
So, for example, if you were born on July 4, your sign is "Cancer," despite the fact that the sun is in the constellation of Gemini on that date. This means that when you check your horoscope, not only are you reading a horseshit prediction, you're reading the wrong horseshit prediction.
"I read your chart right, but you were standing under the wrong sky."