Who renews the insurance. Who holds the mental list of what the children need for Monday. Who hears that the boiler is making that noise again.
Affection tends to be distributed fairly evenly between two people and that work almost never is. The imbalance is rarely decided. It accumulates, one default at a time, until it looks like somebody's personality.
A true partnership isn't measured by how much you enjoy each other's company. It is measured by whether both people can say out loud what the other one is carrying this week.