What breaks personal relationships?
Cheating
Don't feel excited by them anymore
Lack of respect for you as a person/fan
I can't imagine myself supporting any other team the way I support United. So I'm not sure what it would take, but it would be a lot.
What would it take for you to stop supporting your club? Corruption? Relegation?
For me personally it would be corruption in a large scale. Such as bribing refs to win titles. I would find it hard to trust the club after that.