If I understand correctly how game works...

Each tile has a civilian populace attached to it. Its appearance represents cultural/religious affinity as well as it's technological progress(era). If this tile borders another tile with different affinity or from a different era or/and city in general, it will generate unhappiness based on how "far" those cultures are. This can be defined by pre-established cultural wheel or by current game's session events, for example, in this very game player with culture X wage wars against culture Y. With each act of aggression of culture X become more distant from Y. That means the inclusion of a tile with affinity Y into X's city will be harder, for example, it will produce more unhappiness for a longer period of time, or if X's player will continue his aggression against Y's factions/players/tiles will lead to rebellion.
However, if X's decides to work together with Y's culture this tile will require much more time to acquire, but it will generate less "unhappiness".

Speaking of rebellion. Rebels on a spawn acquire unhappy tiles, walling them off of the rest of the city(level 1ish walls), each one spawns a unit.
It would be nice to have "reasons of rebellion", for example, "Y's war of independence on X, because of the aggression" which can be supported by other cultures, nations, affinities, etc. or something like workers' rebellion.