There are a few predominant, definable reasoning errors as to why people fall for the "validity" of Astrology in any aspect.
Confirmation Bias:
Definition; Only taking into account information that verifies a pre-existing belief.
One of the most obvious "mind traps" one can fall into with Astrology is only considering the information that does seem true. When browsing horoscopes or analyzing one's natal chart, it is much easier and more interesting to find the information or predictions which strike us as accurate. Even if the information is overgeneralized, people can take it and interpret it to be personal and applicable. It's very easy to get lost in the few startling coincidences and ignore the larger portion of ambiguity! However, without taking into account all of the misses (or too general of statements to be striking) you are not getting the whole picture of Astrology. The entire point is to attempt to be as overarching and inclusive as possible, to fool you into finding "accuracy"! So watch out!
Availability Heuristic:
Definition; If it is easier to remember, it is more likely true/prevalent.
This may seem a bit of a stretch but reason with me here. Have you ever been asked to describe yourself? It's hard! Turning back to Astrological "truisms" that define one's personality it's a lot easier to define oneself. You can just reach into the prepackaged box of adjectives and slap one on, no qualifications or thought needed, simply by the laundry list given to you by the stars. It's pretty handy dandy in moment's of need to recall "information" about yourself.
Self-Serving Bias:
Definition; Using only information selected because it benefits oneself, at a personal level or otherwise.
Not a lot of Astrological descriptions are framed negatively. Why? Because that way less people would enjoy them! Nobody wants to hear bad things about themselves. Take me, for example. Being the triple-primary-planetary Aquarius I am (just go with me here) I am creative, popular, aloof, flowing, well-spoken, etc etc. Who doesn't want to hear that? Even if it's not necessarily true it's flattering. Even the negatives are put gently--rather than being callous or insensitive, Aquarius is described as "detached" and "individualistic". They mean the same thing, but the connotations are different. Astrology is a form of flattery and self indulgence. It is likely a lot of followers of Astrology believe in it partially for the ego-boost it gives them. This is not to say using Astrology as an ego-boost is bad, only that it doesn't validate Astrology at all.