Unwritten rules are still rules

Some rules are unwritten and emerge naturally over time when people interact with each other. For examples, social norms are often unwritten rules because writing them down would diminish the experience otherwise.

Just those rules they are unwritten doesn't mean you can bend them or conveniently forgotten about them every time. When it's your turn, return a favour, give a gift, make a donation, buy the next round, travel the extra mile, and so on; otherwise, don't be surprised that nobody wants to be in your game anymore.