The forest is a perfect setting for A Midsummer Night’s Dream. The words midsummer and night in the title suggest a story in which nature plays an important role. Midsummer is a time of celebration, warmth, and feasting, and the night can both protect and confound humans. In the play, the lovers Hermia and Lysander flee into the woods from “the sharp Athenian law.” The forest is also a protection for Peter Quince and his band of actors. They prefer to rehearse in the woods at night for privacy and convenience’s sake: “for if we meet in the city, we shall be dogged with company, and our devices known.”
Helena and Demetrius also enter the forest, a place where both danger and desire exist. Lysander wishes to sleep near Hermia (who demurs), and Hermia recounts the times the two “Upon faint primrose-beds were wont to lie, / Emptying our bosoms of their counsel sweet.” Helena claims the woods are not lonely if Demetrius is there. It is a place where even a fairy queen can fall in love with a fool wearing the head of an ass, thanks to the magic of fairies and love. Oberon paints a beautiful picture of the woods where Titania sleeps, a dreamlike “bank where the wild thyme blows.”
However, “the forests wild” can also frighten. Demetrius threatens to assault Helena: “I shall do thee mischief in the wood.” The four lovers get lost and almost come to blows. Even Bottom, whose magically, perhaps grotesquely, transmogrified head terrifies his friends, cannot seem to escape the woods. Interestingly, Theseus and Hippolyta’s escapades into the forest include hunting, a way to express dominion over nature while enjoying its adventure. The mortals eventually return to the city. There, order is restored, but the division between nature and civilization remains tenuous.
The forest symbolizes first and most broadly an alternative to the rational and ordered life of the city.
It is a place where dreams, passions, and magic rule things, and where we lose our established sense of what is possible.
As for why, historically that is one established meaning of the forest; it was the dark unknown, even the sub or unconscious. In the play, this is shown by what happens when the lovers get there.