In Act I, Scene II of William Shakespeare's comedy A Midsummer Night's Dream, a troupe of actors are gathered to discuss and begin preparations for a play to be held on the occasion of the marriage of Theseus and Hippolyta, the duke of Athens and his Amazonian bride-to-be. Peter Quince is handing out acting assignments for the play, Pyramus and Thisbe, to individual members of the cast, among whom is Nick Bottom. Bottom, it will soon be seen, considers himself an actor par excellence, capable of performing virtually every role in the play to perfection. As Quince assigns characters to each of the actors, he approaches Nick and informs him that he, Bottom, is going to portray the character of Pyramus, prompting Bottom's profession of his intent to perform the role, and all others, brilliantly:

That will ask some tears in the true performing of it: if I do it, let the audience look to their
eyes; I will move storms, I will condole in somemeasure. To the rest: yet my chief humour is for a tyrant: I could play Ercles rarely, or a part to tear a cat in, to make all split. The raging rocks
And shivering shocks
Shall break the locks
Of prison gates.

As Quince continues to hand out roles to the other actors, Bottom continues to profess an ability to portray each of those characters flawlessly, as when he urges of Quince,

Let me play Thisbe too, I'll
speak in a monstrous little voice. 'Thisne,
Thisne;' 'Ah, Pyramus, lover dear! thy Thisbe dear, and lady dear!'

This continues, until Quince has finally had enough, telling Bottom:

You can play no part but Pyramus; for Pyramus is a sweet-faced man; a proper man, as one shall see in a summer's day; a most lovely gentleman-like man:
therefore you must needs play Pyramus.

Nick Bottom is an eccentric, egocentric figure. His insistence on acting every role in the play, including that of a lion, reveals not just an arrogant, self-centered character, but a delusional one as well. After all, no one individual can portray every character in a play unless each character is on stage individually, which is not the case in this particular play. Bottom is sufficiently impressed with himself, however, that he is unable to grasp how ludicrous is his vision for the eventual production. Indeed, as the audience will see, so self-centered is Bottom that he remains oblivious to the fact that Puck has replaced his, Bottom's, head with that of a donkey.

He believes himself to be able to play all the parts...a one man show.  We all know folks like him, unfortunately.  This is part of what makes this play so darn funny, and when his head represents his personality and his name later in the play, we all walk away with very sore sides from laughter!

Why does Nick Bottom want to play all the roles in A Midsummer Night's Dream?

The character of Nick Bottom the weaver in Midsummer Night's Dream wants to play all of the parts in the performance of Pyramus and Thisbe that the mechanicals are planning for the Duke's wedding. Bottom is not a mean character, but he does have a very inflated opinion of his own abilities, and he thinks he is quite the actor. He thinks that not only can he do a better job than the other players, he can also do ALL of the parts. Plus, he doesn't much like the idea of Pyramus, who dies for love. He likes to be in control of things, but he really doesn't know what the heck he's doing, as evidenced by the actual performance of the play in Act V.  He does act like an ass, so when Puck sees the group rehearsing near Titania's bower, Bottom is the one he chooses to be the recipient of his spell. 

Nick Bottom is a hilarious character, and it's a good thing that he is just a character. If somebody had a friend like Nick Bottom in real life, it would get old very quickly.

Nick Bottom believes that he should play every part for two basic reasons. First, Bottom thinks that he is the best at everything. He believes that he can play each character better than somebody else; therefore, he believes that he should play every single part. Second, Nick Bottom loves the limelight. He's not a humble character. He thinks very highly of himself and his abilities, and he enjoys strutting his stuff all over the place and telling people how great he is.

He wants to be the center of attention, and playing every part in a play ensures that he gets all of the attention all of the time. The hilarious part about Bottom is that he simply isn't all that great at anything he thinks he's great at doing.

In A Midsummer Night's Dream, why does Nick Bottom want all the roles in the 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!