You are correct about it being cheaper to share the costs of living. However, relationships generally have unique costs we should take into account. You have to invest your time in people (time you might otherwise spend on yourself). Many people feel compelled to spend money in relationships, especially to find and impress the kind of mate they desire. That isn't to say there aren't unique costs in being single either. The point is that relationships aren't obviously as cost-efficient as they may seem at first glance. Sometimes they require real sacrifice.