At some basic level, people within the same society need to agree, at least roughly, on some very basic tenants. The core fundamentals that underpin said society. Things like the value of democracy, freedom of speech, freedom of assembly and others in the case of the US. This does not mean you can not maintain your cultural identity at the same time. The US is very diverse but I bet you if you go from China Town to Little Italy to Appalachia to Harlem to SoHo you would still get all of them to agree on the fundamentals I listed despite the vast gulf between those areas in some very significant ways culturally.