Why do Hermia and Helena quarrel in A Midsummer Night's Dream?

Helena thinks that her good friend Hermia is mocking her by claiming that Demetrius and Lysander are in love with her. After all, it wasn't so long ago that Demetrius was expressing the fervent hope that Helena would one day be killed by a wild beast in the forest. And as for Lysander, he eloped with Hermia, so why on earth would he be interested in Helena all of a sudden?
It's not surprising, then, that Helena thinks this is all a big joke at her expense, and that Hermia is in on it. Helena cannot believe that someone she'd always regarded as such a close friend, someone who'd shared so many special moments with her over so many years, should now turn against her and conspire with Demetrius and Lysander to play such a cruel trick on her.
If there is a trick being played on Helena, however, then it's by Puck, not Hermia. Puck has enchanted Demetrius and Lysander with a particularly potent love potion, though in the case of Lysander it was all a mistake. In any case, Demetrius and Lysander are now both head over heels in love with Helena, and for her, the only rational explanation for this extraordinary turn of events is that everyone's involved in a gigantic conspiracy against her.