Duty and desire have a common source in objective reality. We have the Sehnsucht desire depicted in (or projected from) the Song of Songs because it’s true, noble and right; and being right, it’s also our bounden duty. The common denominator is truth.
God’s unobligated decision to bind himself to covenant love is the basis of our duty to love the one who shows us his gracious favour, but that love which God shows us (what beauty!) also awakens in us the desire to love and be loved. We can in no way exhort ourselves and others to duty and to desire except by a beautiful truth.