In Shakespeare's A Midsummer Night's Dream, why does Nick Bottom want to play all of the parts in the play-within-a-play?

In Shakespeare's A Midsummer Night's Dream, Nick Bottom, the weaver, loves to be the center of attention. When he gathers with his fellow players at Quince's house in Act One, Scene Two, he is eager to derail the casting session with his silly demands. Bottom is happy to be cast as Pyramus in the players' production of "Pyramus and Thisbe," proclaiming that this tragic role will allow him to "move storms" in the eyes of his audience. At the same time, Bottom feels like he would be much more suited to play a tyrant or some role in which he could rant and rave. 

When Flute is cast as Thisbe, Bottom insists that he could play Thisbe as well by wearing a mask and speaking "in a monstrous little voice." When Snug is cast as the lion, Bottom demands, "Let me play the lion too," so that he may roar in a way that does "any man's heart good to hear me" and makes the Duke ask for him to roar again. Quince finally manages to convince Bottom to stick to his own part by claiming that Pyramus is the best looking man of all men. Flattered, Bottom seizes the role. 

This scene makes it clear just how pompous and self-absorbed Bottom is in believing that he could (or should) play all the roles in the play. It's a good insight into his character and a funny setup for the later payoff of the play when Puck turns him into the literal embodiment of his spirit: an ass!