A Midsummer Night's Dream examines the folly of love. In the first scene, we are introduced to Hermia and Lysander, who are in love. However, Hermia's father Egeus wants her to marry Demetrius, a nobleman who also loves her. Meanwhile, Hermia's best friend, Helena, is in love with Demetrius. Helena is jealous that Hermia is receiving of Demetrius's affection. This moves her to tell Demetrius about Hermia and Lysander's secret plot to flee Athens together. Helena means it as a way to earn Demetrius's trust.
In the forest, Puck sprinkles love potion on both Lysander and Demetrius, so when they wake up, they fall in love with the first person they see: Helena. Helena thinks they are mocking her and chides them. When Hermia enters, searching for Lysander, Helena thinks she is in on the plot to tease her:
Lo, she is one of this confederacy!
Now I perceive they have conjoin'd all three
To fashion this false sport, in spite of me.
Injurious Hermia! most ungrateful maid!
Have you conspired, have you with these contrived
To bait me with this foul derision?
Hermia denies this and tells Lysander: "Sweet, do not scorn her so." The spell that Lysander is under is strong, and his "love" for Helena causes him to say hateful things to his true love, Hermia:
Ay, by my life;
And never did desire to see thee more.
Therefore be out of hope, of question, of doubt;
Be certain, nothing truer; 'tis no jest
That I do hate thee and love Helena.
Up until now, Lysander has always professed love for Hermia and was willing to make a plea to both her father Egeus and Theseus, Duke of Athens. When this did not work, he was willing to run away with her, so from Hermia's standpoint, it is odd and uncharacteristic for Lysander to stop loving her—unless someone stole his love. Upon hearing Lysander's words, Hermia believes that Helena has taken Lysander's heart from her:
O me! you juggler! you canker-blossom!
You thief of love! what, have you come by night
And stolen my love's heart from him?
Keep in mind that Hermia has just woken up from a nightmare that a snake was eating her heart while Lysander watched with a smile. She woke up and couldn't find Lysander, so now she thinks that he willingly left her alone in the forest for Helena (which he did, but only because of the love potion). Hermia knows that Helena had been jealous of her earlier on, so perhaps Hermia believes that Helena took Lysander as "payback" because Demetrius would ignore Helena for Hermia.