Oberon asks Puck if he sees that Titania has fallen in love with Bottom.
Oberon and Titania are fighting because Titania has brought a human child, a changeling, back from India. Oberon wants her to give him up, but she refuses. They are at an impasse, and the forest begins to suffer.
Puck replaced Bottom’s head with an ass’s head when he saw Bottom and the other craftsmen practicing in the woods near where Titania was sleeping. He knew Oberon wanted her to fall in love with a beast, so she would realize that the fight between the two of them over the changeling was silly.
When Oberon sees Titania sleeping next to Bottom, a mortal with a donkey head, he feels sorry for her.
Welcome, good Robin. See'st thou this sweet sight?
Her dotage now I do begin to pity (Act 4, Scene 1)
Oberon wakes her up, and she is shocked to see that she was in love with Bottom. She and Oberon make up, and from then on they are in love again and the forest can recover.