Note to Twelfth Night, 2.5.26, "I have heard herself come thus near, that, should she fancy, it should be one of my complexion."

Malvolio is capable of lying to himself, but maybe Olivia did indeed say something to the effect that if she should fall in love, it would be with someone like him. What did she mean? Maybe she was just being nice to her faithful steward. Or maybe, because she was so melancholy about her brother's death, she thought that she might like a man who was, like Malvolio, habitually glum.