Bottom is unconscious of his condition, having just awakened from a pastoral nap in the woods. He thinks the other rustics have concocted a practical joke, at trick to frighten him: “This is a knavery of them to make me afeard.” For even though the rustics have come to the woods to rehearse, they, like all “naturals,” are afraid in the forest – all their folklore has warned them about fairies, sprites, and such other-worldly creatures, and the odd magic they can perform.
And because his friends run away (seeing Bottom strangely transformed), he is left alone in the mysterious place. To assuage his apprehensions, he sings a song of birds, a song which draws Titania’s attention to him. She, of course, is under a spell to fall in love with the first person she sees. So the author’s purpose is to justify the meeting of Titania and Bottom without anyone else being around, and, of course, to equate the look of the ass with the behavior of an “ass.” The comic juxtaposition of the two “worlds,” the supernatural (or subnatural) and the low-life real world, was comic gold in the Elizabethan age; Shakespeare in his genius then adds the other real-world of Theseus, Hippolyta, and entourage (again, an echo of the ancient Greek world.)