It not only causes us to be unhappy, but can strain or ruin relationships, distract us from work and family and other important things, make us reluctant to open up to new things and people.
A century and a half later we find ourselves in the world he anticipated, where everyone's life is experimental and provisional, and sudden ruin can happen at any time.
Since the beloved is a major source of happiness, this person is also a major threat to our happiness: more than anyone else, the beloved can ruin our happiness.