My dad used to tell me, two can live as cheaply as one.
I took his word for it, because I saw the principle operate around me every day. Our family was willing to share, and we were willing to make compromises with each other to make it possible to share.
One of the big compromises was rules and habits. By sharing rules and habits, we were able to eliminate a lot of waste. Eight people were able to live on an untenured professor's meager salary by handing clothes down, by sharing cleaning and cooking duties, by sharing space, by sharing from the common pot.
I have since had occasion to see many couples who can't seem to even live as cheaply as if they were two people living alone. For them, two can't even live as cheaply as two.
Why?
Not only will they not share clothes, each insists on having more clothes than the others.
Not only do they not share cleaning duties, but they insist on making more of a mess for the others to clean up.
Not only do they not share cooking duties, but each insists on using all the pots, all the burners on the stove, the oven and the microwave, etc., then leaving their cooking on the stove and in the oven, not just to cool, but because they haven't got the time to put it all away and get it out again for the next meal.
Not only do the not share space, but they insist on setting up no-man's land between their own space and everyone else's.
And not only do they not share what food they have, but they take food that others have cooked, put it on their plates, and then, instead of eating it, complain that it is not to their own liking.
I think it all boils down to their wanting to get attention, but not being willing to give attention.
