Why does Demetrius dislike Helena so much before Puck uses the flowers juice on him?
Helena reminds Demetrius that he used her and dumped her for the prettier and better socially placed Hermia. He made promises to Helena that he could not keep, so he doesn't want to be reminded of them. Shakespeare also intimates that he took Helena's virginity when he has Lysander say that Demetrius "made love" to old Nadir's daughter Helena. Helena would have expected that Demetrius would marry her after she had given herself to him. Instead, he gets engaged to Hermia and avoids Helena. He would not have liked it that she still proclaimed her love for him every chance she got.
Because if we analyse the text A Midsummer nigth's dream
it's clear that Demetrius doted on Helena in the beginning but
later he dislikes her the reasons may be as follows
1-he met Hermia more beautiful than her
2-Hermia's father's consent for him
and it may be because if we annalyse the characters of Hermia and Helena restricting our selves to the text it's obvious that Hermia is more beautiful attractive tender and etc than Helana