'Tis but fortune; all is fortune Malvolio is giving himself a little pep talk. 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 she's a countess, and he's only her steward, fortune could make her love him.