Instinct, impulse, reaction, whatever you may term it, instant behavior on first contact is just human. Just like computers have been programed to execute operations in a familiar and orderly process, the human brain functions in a way it has been nurtured to perceive, adapt and process information. We have certain expectations which assumes thing in a phenomenal manner, but if they manifest outside the confines of our orderly understanding, the brain sends a mixed signal which results to unclear judgment. This is my understanding on the psychology of the brain. Going back to the topic....
...."DO NOT JUDGE A BOOK BY IT'S COVER"
This is probably one of the most common and familiar quote known to everyone but unfortunately, a large number of the populace fail to reckon with it's meaning in their day-to-day lives. It is perhaps the only trait or characteristics every human personality shares and have in common...and it is because no matter how different our personalities are, our brains function the same with exception of the mentally disturbed.
I can't say I fall into the many who are guilty of judging people by first contact, it's because am an introvert. On the contrary I fall into the category of the judged on first contact, a very unpleasant feeling.