4 ideas
Most people follow their society's unwritten rules by instinct, without knowing they are doing it. The rules only become visible when someone breaks one on purpose and others react, for example by bumping into strangers and counting how many of them say sorry. Seen this way, a culture works like a grammar: a shared set of codes that runs across class, age, sex and region.
Talking about the weather is less about weather than a code for getting past the awkwardness of speaking to someone. It works as a greeting with a set reply: you are expected to agree, and if you disagree you must present it as your own personal quirk. How the other person answers tells you whether they want to chat or are in a bad mood.
A culture can set aside particular places or substances where its normal restraints are lifted. The pub bar is the one place where talking to strangers is allowed, and alcohol is used to loosen people up and bond them. Even this freedom has rules: those conversations follow strict and quite complex codes of their own.
Dry, ironic humour runs through nearly every kind of social occasion, from small talk to serious work. It lets people say things while keeping a safe distance from sincerity, which suits a culture that is uneasy with open feeling.