Why does Nick Bottom want to play all parts?

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.