Nag

Nagging is caused by not getting your own way, for both parties.

Not getting your own way all the time is the price you pay for shacking up.

There is always one who takes on the nagging role. And one who is the naggee.

It’s a natural polarization effect after two people shack up. 

Initially they sometimes start very similarly.

But one nags more than the other and the other gets more offended and thus the polarization begins.