Note to Twelfth Night, 2.5.23, "'Tis but fortune; all is fortune"

Malvolio is giving himself a little pep talk. He tells himself that the only reason that he's not already married to Olivia, and in control of all of her wealth and status, is "fortune" (chance, luck); he just happened to be born in a lower station of life than Olivia. On the other hand, as he says a little later, things could change; fortune could bring him Olivia, because "the lady of the Strachy married the yeoman of the wardrobe." In other words, despite the fact that Olivia is a countess, and he's only her steward, fortune could make her love him.

All is Fortune