Annexing people makes people upset!

I want to understand the mechanics and reasons why homestead and town annexations were added. Isn't this game about cooperation? I've found that people who get annexed leave the game quite often and low population is already an issue. So what are the reasons behind this if you can't play a diplomat/culture war and try to take over other cities and countries to enforce your laws? This game isn't like real life, where you can't just 'quit' so why make people upset with such game mechanics? And is there a way to make it a 'friendly annexation'?