Science still won't explain the mysterious nature of love and despair.
Despair is part of love.
Depression is the flaw in love. There's no such thing as love without the anticipation of loss. And that specter of despair can be the engine of intimacy.
Depression is the flaw in love. To be creatures who love, we must be creatures who can despair at what we lose, and depression is the mechanism of that despair.