Scale

Social interactions can remain intuitive while every person in a group knows each other (cf "Dunbar's Number"). When an effective social network grows past this number (such as in towns or cities), social interactions with strangers must occur. These 0-(initial)-knowledge interactions must be mediated by some kind of social convention so that interactions to not lead to broken expectations (and therefore trigger anger and likely conflict).

 Negative and Positive Expectations

Any expectation can be negative (an expectation that a person will not do something) or positive (an expectation that they will). Positive expectations are highly unstable -- even if a person would like to perform the expected action, they may have any number of higher priorities. Positive expectations also compete; since people generally perform one conscious action at a time, multiple positive expectations are generally quite challenging to simultaneously guarantee. Even if the positive expectations are only conditional, they can easily co-occur and conflict (eg "when it's sunny, I expect people to say hello to me").

Negative expectations have no such conflict. Not doing one thing in general makes it no harder to not do something else. Negative expectations also do not conflict with spurious priorities, unless the priority is actually in direct contradictions.

While negative expectations only conflict with eachother or with priorities in sparse situations, positive expectation conflict is dense, and tends to provide intractable expectations.

Sidenote: Continuous Taxation

Continuous taxation is an example of a formally positive social expectation which is constructed so as to be pragmatically no expectation at all, which then sitesteps the above issues. Continuous taxation, i.e. taxing a sale during a transation, or taxing a paycheck before it reaches the employee, does not require any additional steps on the part of the buyer or employee, and as such there are no pragmatic steps which individuals must consciously undertake.