What is interesting about the character of Demetrius at the beginning of the play is the way that he is made out to be an inconstant heartbreaker who has cruelly spurned Helena in exchange for his pursuit of Hermia. Note what Lysander tells us about Demetrius and his relationship with Helena:
Demetrius, I'll avouch it to his head,
Made love to Nedar's daughter, Helena,
And won her soul; and she, sweet lady, dotes,
Devoutly dotes, dotes in idolatry,
Upon this spotted and inconstant man.
Clearly, in spite of the obvious bias that Lysander has against Demetrius, this does serve to present Demetrius as a somewhat "inconstant" individual who has cruelly broken the heart of Helena by being with her and then leaving her for Hermia. The way in which Demetrius therefore, through magical enchantment, returns his affections back to Helena and away from Hermia in Act 3 scene 2 is rather amusing because suddenly he is in the position of loving in vain. His sudden protestations of love to Helena, whom he had equally violently despised just a few scenes ago, clearly present him as a ridiculous individual who allows his emotions and his feelings to completely overwhelm his reason. This is of course one of the key themes of this play that Shakespeare seeks to develop. The humans do not come out of this play very well!