When developing a culture, a lot of thought is often put into the religion, gender structures, hierarchies, and education of the society. One often ignored issue that can provide a lot of story hooks and conflict between people is that of social ettiquette (I'll call it customs from now on for brevity's sake). Customs can vary widely - particularly in places where travel is difficult - and there are variations within certain sub-cultures (i.e. poor vs. rich, military vs. civilian, young vs. old).
Of course, for us fantasy writers, we have to first develop these customs before we can see where the conflicts might lie and that can be rather daunting. There's a huge array of social customs that you can develop (greetings, insulting behavior, gift-giving, courting, superstitions, stigmas, farewells). Even a society that appears on the service to be rough-and-ready or laid-back will have its own set of social rules. Generally these customs are a social lubricant, providing a shared set of norms and assumptions, to make it easier for people to relate to each other and anticipate behavior.
This makes it sound easy. Just invent a set of social rules, watch them clash, and that's that. However, customs aren't arbitrary. If you remember my earlier post on symbolism, a society's customs are often steeped heavily in what is considered symbolically 'good' or 'bad'. A society that values a non-confrontational attitude will have quite different customs to one that values exuberance and emotional forthrightness. Why are certain things considered inappropriate? What does this represent to them?
Even a single social rule can have a multitude of reasons. You don't have to accept the first answer you come up with. If it still doesn't fit right, brainstorm a little. What other possibilities could there be? Do certain sub-cultures respect the same custom but for different reasons?
As an example, one of the customs in the Realms is that you don't complain about the rain. The possible social reasons behind this might be: It'll anger the sun spirits; It goads the rain spirits to keep raining; Rain is depressing and complaints about depressing subjects bring everyone down; Any and all complaints are considered little more than worthless whining in a stoic society; or even Wishes might come true and a drought-stricken land needs all the rains it can get. In the Realms, it's actually the latter reason. One of the cruelest things you can say to a Realms person is: 'May it never rain on your city's/region's celebrations' because the Realms will use anything as an excuse to celebrate and if that came true then there would be a year long drought!