What is it?

What is it that attracts us to some people from the first moment we see them, and makes us not like some other before we even talk to them?  Has it ever happened to you?  It has to me.  There are a few very good friends I have now that before I knew their names, before I had spoken even one sentence with them, the first time I saw them in a party, in school, or at work, I knew we were going to be part of each other's lives, because we both wanted to learn more about each other, keep in touch, and be friends.  Then, there are other people who might as well be nice, caring, wise, etc., but you do not feel inclined to invest so much time and energy in having a relationship with them; no matter how hard they try.  You do not know the reason, but based on my past experiences, often times something happens later confirming your initial gut feeling and intuition about that person, both positive and negative.  In fact, it has worked so well for me that I trust my gut feeling and intuition very much.  It has never lied to me.  Or does it lie to us and we unconsciously look for reasons to confirm our beliefs based on our intuition and we ignore other data that disapproves it?

Another example is in dating.  I read somewhere that within the first five to seven seconds of a first date, people have already decided whether they like to see the other person again and go for a second date or not.  That is why these speed dating and four minute dating businesses are flourishing.  If your first impression is negative, during the rest of the date you are acting nice and polite while you do not really want to be there.  How are we humans communicating these perceptions and feelings among ourselves?  Are we sending and receiving some information through some form of invisible signal?  Do we really have such sensors?  Is there a scientific explanation for gut feelings and first impressions? Or is it that we are simply inferring so many things from the way a person dresses, smells, talks, walks, sits, stands, listens, and behaves?