It is a standard that we hold them accountable to: when they fail to act normally, we demand an explanation, and we break off relations or otherwise punish them if they are unable to supply a satisfactory one.
One very important point about 'being normal' is that it significantly reduces the cognitive strain that one's behavior puts on other people. In a typical social situation- say, walking down a city street- there is simply too much going on for anyone to pay proper attention to all of the possibilities...
That is why the culture shock that people experience in new or unfamiliar social environments is such a consistent and well-documented phenomenon. Culture shock is, in essence, the effect of cumulative frustration and anxiety stemming from the loss of the familiar signposts used to manage social interactions...
Garfinkel concluded that the dense network of rules that govern our daily lives serves as the 'routine ground of everyday activities'. Its most important function is to maintain and reproduce a system of generalized trust...
What we need to believe is that others will obey the rules."
