Demetrius is caused to fall in love with Helena in Act III Scene 2 when Puck anoints his eyes with the love-flower. He is then awakened by Lysander and Helena. Lysander is busy protesting his love of Helena, and Helena thinks that he is joking and then begins to get angry with him. It is significant that Demetrius wakes up to spy Helena (and therefore fall in love with her) after Lysander has said this line to Helena: "Demetrius loves [Hermia], and he loves you not!"
What irony it is then that Demetrius wakes up at that point to contradict Lysander and his love pleas by protesting his undying love and affection for Helena with these words:
O Helen, goddess, nymph, perfect, divine,
To what my love, shal I compare thine eyne?
An important question that must be asked is how real are Demetrius' emotions? This is not a reaction that has come from within him - rather this love is induced by an outside agent - namely Puck and Oberon. Demetrius is the only character who is not changed back at the end of Act IV to be like he was, and therefore some critics have suggested that there is something unreal about his love for Helena.