Puck is talking to Lysander and Demetrius (although they cannot hear him) as he plans to get them each tired and lost in the forest so they can have magic worked upon them when they eventually drop with tiredness. He imitates the voice of Lysander when talking to Demetrius and vice versa. This is a particular delight to Puck and he enjoys it like a great game. He speaks of it in this instant as though he were describing the fun of what he will do and what fun it will be (for him!). The reason for his doing this is because Oberon has given him instructions to douse Lysander and Demetrius with magic, and he needs to separate them and get them tired in order to accomplish this.