Why do we instantly like or dislike certain people?

In this discussion thread I don’t want to deal with racism, hate groups, or group inclusionary/exclusionary psychology. Nor do I want to deal with situations in which we might naturally distrust another person, e.g. being followed by a strange man while walking down a dark alley.

I want to deal directly with why we seem to instantly like or dislike a waiter, a clerk at the checkout counter, or a new neighbor or coworker.

We recently added a new employee at the office. He is smart, well dressed, clean, polite, and responsible—but I don’t really like him, and I don’t know why. He has not said or done anything wrong, and he doesn’t remind me of anyone I hate. I don’t want to say that I hate him; it is more like a slight distrust of him, and a moderate amount of disinterest in him as a person—and I’ve felt like that from day one.

I consider myself to be a fairly decent judge of a person’s honesty or legitimacy. For example, I can usually spot a liar or con man pretty quickly. I also consider myself to be pretty accepting of differences in people, and have deliberately exposed myself to all kinds of cultural experiences. I’m also not a tribalist—a person who seems to have an aversion to everyone outside his group. So why do I—like many people do—instantly like/trust or dislike/distrust another person?

Is it simply gut instinct? In other words, is it some kind of survival device that evolution has equipped us with? We all know that being able to make rapid assumptions about the possible threat of other life forms or situations is key to survival. We make rapid and reliable assumptions all the time. But are these instantaneous responses (in relation to like/dislike) nothing more than a left over—and misplaced—instincts from our cave dwelling days? And if they are, can we trust them?

Should we try to overcome these inclinations? Is it fair to other people?

What do you think?