In many ways, Shakespeare portrays the chaos of abandoning reason completely in many characters throughout A Midsummer Night's Dream. The play does often show frequent switches from desire to desire; as Bottom tells us, "the truth, reason and love keep little company nowadays" (3.2.127–128). Just as Puck claims "what fools these mortals be" in the final act of the play (3.2.115), Shakespeare makes a complicated argument that reason is absent in human affairs.
All of the characters throughout the play are deeply irrational, as we can see when they frequently change the object of their desire from person to person. Much like the effects of Cupid's arrow in Greek and Roman mythology, Puck's love potion continuously changes the characters' desires, and characters constantly make their decisions with reckless abandon. Demetrius, Lysander, Helena, and Hermia all constantly shift their desires for each other, due in no small part to Puck dropping the fairies' love potion in their eyes. Even before Puck's love potion, Helena explains theses irrational effects of love during her soliloquy in act 1, scene 1:
Love looks not with the eyes but with the mind,
And therefore is winged Cupid painted blind.
Nor hath love's mind of any judgement taste,
Wings and no eyes figure unheedy haste.(1.1.234–237).
Helena clearly states that the logic of love is contained in the mind, which is itself "blind." She contends that "love's mind" has no trace of judgment and that, given this, it symbolizes "unheedy haste." "Unheedy" is an incredibly rare and antiquated word meaning "unheedful" or "careless" (OED). Helena's explanation alone shows how Shakespeare represents the lack of reason in most characters' desires and conceptions of love throughout A Midsummer Night's Dream.