why are Demetrius and Helena in the woods?

Demetrius and Helena arrive in the woods right next to where Oberon is hidden. Demetrius tells Helena to go away, and that he does not love her even though she has told him about Hermia and Lysander trying to run away. She threatens to chase him down if he should try to leave her in the woods. Oberon, having overheard the entire conversation, decides to make Demetrius fall in love with Helena. He tells Robin Goodfellow to take some of the juice and go anoint the eyes of the Athenian man in the woods, but doing so only when it is certain that the woman by his side will be the first person he sees. The puck agrees, and goes off to carry out his errand.