The reason we like to have a balanced relationship with others—where we put in as much effort for them as they do for us—is probably because of the pain we feel when we’ve invested a lot in a friendship that then disappears before our eyes. When someone doesn’t make an effort for you, it’s a sign that they don’t value the friendship much, meaning it’s fragile. We sense that fragility, and that’s why we also lose the desire to make an effort for them.