"Some are born great, some achieve greatness, and others have greatness thrust upon 'em."

Malvolio:
"'M.O.A.I.' This simulation is not as the former, and yet to crush this a little, it would bow to me, for every one of these letters are in my name. Soft, here follows prose." (II.v.143-6)

(How is M.O.A.I. a code for Malvolio? The letters are not in order. Why would he assume it's him?)

Here, Malvolio is reacting to a particular selection from the note from Olivia that he found. Because of the content of the note, believes that Olivia is in love with him. Malvolio is ecstatic. However, this particular part of the note causes him concern...temporarily, at least.
He recognizes that the letters in the note (M.O.A.I.), although part of his name, are out of order. Nevertheless, Malovlio convinces himself that this letter is addressed to him. Malvolio, now under the impression that Olivia loves him, discovers his own mutual love for Olivia. When he sees the letters M.O.A.I., he ignores that they are out of order ("the simulation is not as the former") and tells himself that with some simple rearranging ("crush this"), these letters clearly spell out part of his name (the letters "would bow to [him]", giving him what he wants - evidence of Olivia's love). It's more like wishful thinking - Malvolio REALLY wants this love to be true, so he will look for any way possible for these letters to be related to his name.