The mood in Act III of A Midsummer Night's Dream is passionate confusion. It is the end of a trying day for the four lovers, Hermia, Lysander, Helena, and Demetrius, all of whom are now in the forest. Lysander and Hermia have run away because of love, Demetrius is seeking them because of love, and Helena is chasing him because of love. In the woods at the same time are the players, come to rehearse their offering for the Duke's wedding, and, of course, the fairies.
Shakespeare uses Puck, acting on the orders of Oberon, as the device for plunging all these characters into confusion about who they are, who they love, and what is and is not real. It is as though he has taken the inner turbulence caused by the "Love" state and placed it on the stage for us to witness, in all its alternating certainties, conflicting emotions, and wild irrationalities. The mood is one of confusion, rapid shifts in emotions and attractions, and expresses, through this device, the very nature of what it's like to be in love.