Lysander and Hermia represent a couple who is physically devoted to each other but is also not the strongest of couples. We see their weaknesses as a couple when they encounter obstacles, such as enchantment, and their relationship quickly crumbles. The unstable relationship of Lysander and Hermia is important to the story because it helps portray Shakespeare's themes concerning human folly and the irrationality of love.
We first see Lysander and Hermia's devotion to each other in the first scene when we learn that Hermia is being commanded by her father to marry Demetrius instead and is petitioning Theseus for Hermia's death should she continue to disregard her father's wishes. While Demetrius and Egeus are talking privately with Theseus, Lysander and Hermia decide to elope instead. They plan to cross through the woods to Lysander's wealthy aunt's house where they can marry without the "sharp Athenian law" hanging over their heads and threatening Hermia's life (I.i.164).
We further see just how strongly their relationship is based on physical attraction when we see Lysander try and seduce Hermia in the woods. Like a caring gentleman, when Lysander sees that Hermia is tired from their travels, he suggests that they sleep in the woods for the night and carry on in the morning. Like a virtuous maiden, Hermia agrees, but tells Lysander not to sleep next to her. However, Lysander shows his sexual desire and presses her, trying to seduce her with his poetic words by saying, "One turf shall serve as pillow for us both; / One heart, one bed, two bosoms, and one troth" (II.ii.42-43). Lysander's show of sexual desire for Hermia demonstrates that their relationship is strongly based on physical attraction.
Since their relationship is strongly based on physical attraction, the audience is not surprised when Lysander is so easily distracted by the magical flower into falling in love with Helena instead. The sudden switch in Lysander's affection helps portray Shakespeare's themes that mankind is foolish and that love is irrational. We see these themes portrayed in Lysander's lines to Helena, "The will of man is by his reason sway'd, / And reason says you are the worthier maid" (II.ii.117-118). These lines are ironic because in actuality Lysander is not using his rational mind to determine his new found affection for Helena but instead basing his interests on physical attraction, just as he did with Hermia. Shakespeare is using these lines to point out that the flaw of mankind is that man is actually very irrational and that love is an irrational emotion.
Hence, we see that Shakespeare uses the physical relationship of Lysander and Hermia to say that mankind is foolish and that love is an irrational emotion.