We especially see Puck follow his heart when he reaches out in sympathy to whom he thinks is Helena. Oberon has just witnessed Demetrius being cruel to Helena in the woods and has commanded Puck to use the flower on Demetrius in order to make him fall in love with her, thereby mending her broken heart. Unfortunately, the only guidance Oberon gives Puck for finding Demetrius is that he "shalt know the man / By the Athenian garments he hath on," not knowing that there are actually two Athenian couples in the woods that night (II.i.269). When Puck finds whom he think is the correct Athenian couple and sees the fair maiden sleeping so very far away from the gentleman, he assumes that it is because the man is being cruel to her, rather than that the woman is trying to preserve her maidenhood. Puck's heart genuinely goes out to the woman in sympathy, and he chastises the man, saying:
Churl, upon thy eyes I throw
All the power this charm doth owe:
When thou wakest let love forbid
Sleep his seat on thy eyelid. (II.ii.78-81)
In calling whom he thinks is Demetrius a "churl," Puck is calling him a low and insensitive person. His chant is also promising that once the man awakes and lays eyes on whom Puck thinks is Helena, love will never let the man sleep again. Hence, we see that Puck follows his heart in amending what he thinks is a terrible situation and by doing what he thinks is rescuing a maiden from cruelty.
In contrast, Puck follows his prankster instincts rather than his heart when, after realizing he has enchanted Lysander to fall in love with Helena rather than Demetrius, according to Oberon's instructions, he brings Helena to Demetrius, but with Lysander in tow. After Oberon sees how Puck has broken up a true-love relationship rather than mending a broken relationship, he commands Puck to bring Helena to their part of the forest while Oberon enchants Demetrius. Puck successfully does this, but Lysander follows after her. Puck thinks it is great fun to now witness both men fighting over the same woman, as we see in his lines, "Shall we their fond pageant see? / Lord, what fools these mortals be!" (III.ii.115-116). Oberon accuses Puck of having brought Lysander with Helena on purpose thereby inciting a ridiculous fight among all four Athenians, as we see in Oberon's lines, "This is thy negligence. Still thou mistakest, / Or else committ'st thy knaveries willfully" (III.ii.361-362). Since it can be said that Puck brought Lysander with Helena so he could observe their fight, thinking it great "sport," we can say that in this moment Puck was following his mischievous instincts, rather than his heart.