Puck's spirit, or energetic, devious nature, dominates the playful, comic mood of the play through his mischievous, manipulative antics. His final lines of the play particularly create a very comic and joyful mood.
We learn that Puck is a particularly devious and mischievous fairy from his fellow fairy's comments about him in the first scene in which he appears. His fellow fairy asks him if he is the "shrewd and knavish sprite" who frightens village maidens, steals cream, messes with the grinder in the mill, as well as other devious antics (II.i.33-39). The term "shrewd" can be translated as malicious or spiteful while "knavish" means "mischievious," showing us just exactly what a rogue little fairy Puck is.
While Puck is indeed mischievous, he genuinely mistakes Lysander for Demetrius as Oberon only tells him that he would "know the man / By the Athenian garments he hath on" (II.i.268-269). Puck had no idea that there were two Athenian men in the forest that night. His simple mistake sets the tone for the entire play, which is both comic and playful. He adds to the comedy and the irony of the situation by letting it be known just how much fun he is having in viewing the consequences of his error, as we see in his lines:
Then will two at once woo one.
That must needs be sport alone;
And those things do best please me
That befall preposterously. (III.ii.119-122).
In this passage, the term "sport" can be translated as "pleasant pastime," showing us that he thinks viewing the two men fight over the one woman and the two women fight with each other is a great deal of fun (Random House Dictionary). Also, the term "preposterously" can be translated to mean "foolishly," showing us just how foolish he thinks the humans' behavior is.
Finally, at the end of the play, Puck further sets the final, satisfying, happy mood of the play by showing how the fairies have made amends for their antics, especially his own antics, as we see in his final speech:
Gentles, do not reprehend.
Give me your hands, if we be friends,
And Robin shall restore amends. (V.i.425, 432-433)