How and why does puck change bottom?

Bottom is blissfully unaware that he is transformed into an ass, and humorously asks the others why they run away from him.At this point Titania wakes up and sees Bottom, with his asses head, and falls in love with him. She begs him to keep singing and making jokes for her, and entreats him to remain in the forest with her. She then calls four fairies in to take care of Bottom and lead him to her garden.