Hermia and Helena have a very strong friendship. It's the kind of friendship that endures through thick and thin, throughout life's many ups and downs. And there are certainly more than a few of those in the play.
The biggest problems that the close friends encounter relate to love. Friendship may be powerful, but as Hermia and Helena discover, love can be even more so.
That love should complicate their friendship shouldn't come as too much of a surprise, given how many complications it's already caused. Hermia is in love with Lysander, and this causes her no end of trouble with her father, Egeus, who steadfastly refuses to let them get married. In response, the young lovebirds show their romantic side by running off to the forest together. There, they will be far away from Egeus as well as Demetrius, the man whom Egeus wants his daughter to marry.
If this weren't complicated enough, the mischievous sprite Puck arrives on the scene and proceeds to throw a gigantic monkey wrench into Hermia and Helena's friendship. By mistake, he squirts a love potion into Lysander's eyes, ensuring that when he wakes up, he'll immediately fall head over heels in love with Helena. Puck had mistaken Lysander for Demetrius, but in any case, the damage is done.
As for Helena, she is in love with Demetrius, the man Egeus wants to marry his daughter Hermia. Her love for Demetrius, as well as her absolute loyalty towards him, comes between herself and her best friend; Helena betrays Hermia by telling Demetrius of Hermia and Lysander's plan to elope to the forest.
It is unlikely in the extreme that Helena would have done such a thing had she not been in love with Demetrius. As it is, she's jeopardized her friendship with Hermia over a man. To make matters worse, Hermia is furious with her best friend when her beloved Lysander turns his attentions towards Helena.
Thankfully, all is resolved in the end; Hermia marries Lysander, and Helena marries Demetrius. When all's said and done, however, there's no doubt that Hermia and Helena's friendship has really been put to the test.