Loving someone who causes you unhappiness
At some point in a relationship, something the other person does will cause you to feel unhappy. With our parents, perhaps it is their advice to our spouse, as to how thinks should be. Even though you are content with the way your spouse treats you, or does the topic being advised on. With children, perhaps it is who they choose to date, or maybe what they decide to study for preparing for a career. With our spouse, perhaps they are not driven enough, to climb the corporate ladder. Regardless of what the situation is, we need to love the person as they are, and be happy with them as they are. Learning to love without conditions, being totally accepting, not becoming angry or resentful, is a major step on the dharma path.