Reason is the law which, as Duke of Athens, he must uphold. Desire is the lovely and desirable Hippolyta, who is clearly disappointed in Theseus' stern reading of Athenean law. Before the scene is over we have the introduction of yet another character, the beautiful Helena, who complains to Hermia that she is in love with Demetrius. This is great by Hermia who wants to get rid of Demetrius so that her father may relent and let her marry Lysander. We might also note at this point that there seems a definite relationship between love and rejection.
Love only flourishes when one or both of the parties overcome something.
Love must be won. Theseus has apparently overpowered Hippolyta. Lysander and Hermia are attempting to overcome Egeus' edict, but there is nothing to overcome for Demetrius. Helena's open pleading for his love is too easy, so it does not develop. The scene ends with all four of the unhappy lovers planning to meet that night in the wood on the edge of Athens.