P19: He who loves God/Nature cannot strive that God/Nature should love him in return.

Dem.: If a man were to strive for this, he would desire (by P17 C) that God/Nature, whom he loves, not be God/Nature. Consequently (by IIIP19), he would desire to be saddened, which is absurd (by IIIP28). Therefore, he who loves God/Nature, etc., q.e.d.