How? He had to put up with that a$$hole Arthur.
Arthur was annoying but harmless.
I can't even list all the fukked up shyt Doug did throughout the marriage but he constantly lied pretty much every episode, manipulated every situation to his benefit (And often times screwing over his wife in the process), literally had zero ambition to ever better himself or their situation, and was a hypocrite. One example, Doug got mad and was going to divorce Carrie on the last episode because Carrie kept the apartment in Manhattan, when he literally did the exact same thing earlier that season, and all she did was get somewhat agitated and then forgave him within a minute.