SAD as in Social Anxiety Disorder is one of the explanations for why people are shy. It’s an appropriately named disorder because it co-exists with depression a lot. SAD probably has some neurochemical basis because if you give antidepressents–which are drugs that cause global and somewhat random modifications to your brains neurochemistry–people with SAD sometimes get better. Everyone’s brain is neurochemically unique and one drug won’t work as well as other. Some people with SAD use alcohol successfully or doctor perscribed antiaxiety drugs to the same effect. Personally, drinking has no impact at all on my social inhibitions. After a few beers I still really don’t want to talk to people I don’t know very well, but I now have to concentrate on walking. I read somewhere (but was too lazy to keep the reference) that this phenomenon is commen enough for researchers to notice that alcohol doesn’t alway help people with SAD relax their ‘inhibitions.’
SAD means being normal & sociable with people you already know, but very slow and cautious around people you don’t know. It probably is a behavioral characteristic common to all mammals, just like bossiness–being an alpah male, territoriality, and a host of other common behaviors. The tuning of this part of our brain determines if we are asocial (sometimes gets grouped under avoidant personality disorder) or at the opposite end of the spectrum where everyone we meet we classify as a friend, including strangers who really don’t have your best interests at heart, like insurance salemen and lawyers.
There are lots of other reasons why someone might be shy or have few friends. Someone with a autism spectrum disorder might just have a poor model of what other people are thinking–for example, someone with aspergers or autism might not realize that no one else is as passionate about industrial frying equipment as they are. I have a relatively good model of what people are thinking, but tend to make good use of it when I’m around people I already know. Social success relies on a lot of skills and preconditions, anywhich could be missing for a variety of reasons, so like any mental state that gets in the way of your goals, be cautious of misdiagnosis.
There is also some overlap with our biological mechanisms for fear and fear of punishment. Our limbic system is one of the more ancient parts of our brain that allows us to skip thinking about the danger at hand and go directly to fighting or running away. In a SAD person, this translates to avoidant behavior, leaving a party early, and unfriendly conversation. To further aggrivate matters, there is a pretty clear correlation between social situations and having the limbic system going haywire, so a SAD person can expect to see negative reinforcement for attempting to deal with strangers.