In what ways does Puck follow his heart in A Midsummer Night's Dream?
I rather think how you answer this question depends on your own personal ideas about Puck and the kind of role he plays in the play. This is of course where subtext comes in: how we interpret the lines that we are given of the play. We must recognise that directors who have staged the play have produced very different kind of Pucks over the years.
My own feeling about Puck has always been that he is a force of chaos in the play who delights in making things more complicated than they actually are. If I were a director, I would show this by having Puck deliberately confuse the Athenian lovers to create a more complicated situation for his own amusement. We can see his delight in causing chaos especially when he transforms Bottom and then chases away th other players in Act III scene 1:
I'll follow you, I'll lead you about a round,
Through bog, through bush, through brake, through brier.
Sometimes a horse I'll be, sometimes a hound,
A hog, a headless bear, sometimes a fire...
We can see the intense enjoyment that Puck takes in scaring the players and frightening them away.
In addition, note how Puck is pleased by the prospect of both Lysander and Demetrius being in love with Helena after Demetrius has had his eyes annointed in Act III scene 2:
Then will two at once woo one;
That must needs be sport alone.
And those things do best please me
That befall preposterously.
Puck loves things that "befall preposterously," as they provide him with intense enjoyment. All of this points towards the way in which Puck most definitely follows his heart in this hilarious comedy. He is a force of chaos, and he acts to be true to this.