In Act 2, Scene 1, Puck fetches a pansy (a.k.a. "Cupid's flower") so that Oberon can use its magic juice to make his victims fall head over heels in love. Here's how Oberon describes it:
The juice of it on sleeping eye-lids laid
Will make or man or woman madly dote
Upon the next live creature that it sees (2.1.8)
The stuff actually works and it wreaks havoc on several characters. After Oberon drops the love juice in sleeping Titania's eyes (2.2), the Fairy Queen wakes up and falls in love with an "ass" (3.1.1). Puck also squeezes the love potion in Lysander's eyes and, when he wakes up and sees Helena, Lysander forgets all about his girlfriend and becomes fixated on Helena. This goes on and on until Oberon and Puck take pity on their hapless victims and whip out an antidote, which is the "juice" of a different kind of flower – "Dian's bud" (2.1, 3.2, 4.1). The love juice is a lot like Love Potion Number 9.
Why does this matter? Well, the juice's fast-acting power seems to mimic what often happens in real life. As every hormone-driven teenager knows, love can be unpredictable and inexplicable. Falling in love often happens in an instant, without warning, and falling out of love can happen just as fast. We talk about this more in "Themes: Love."