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The plot thickens quite quickly in terms of Helena's position. She is desperately in love with Demetrius who was supposedly in love with her
.. he hail'd down oaths that he was only mine (I.i.242)
until he saw Hermia at which point he only had eyes for Hermia. It is especially hurtful to Helena as the women are best friends. She even tries to win Demetrius back by telling him of Hermia's love for Lysander but it backfires on her as it strengthens his resolve.
Helena is somewhat obsessive in her pursuit of Demetrius and feels that love is cruel and Hermia seems to have everything going for her.
Things base and vile, holding no quantity,
Love can transpose to form and dignity.
Love looks not with the eyes, but with the mind;
And therefore is wing'd Cupid painted blind.
After Puck puts love juice on Demetrius' and Lysander's eyes and they both profess their love for Helena, she does not relish it but thinks the three, Hermia included are mocking her and playing tricks on her, further strengthening her enviousness of her friend - that she would be in such a position.
Refer to the eNotes study guide and navigate to the character analyses to understand Helena better.
helena is envious of hermia because she loves demetrius and he is fawning over her best friend who doesn't even like him, after he proposed his love for her.