It might make more sense if you substitute the words "selfishness" with "self-interest." Selfishness has come to represent a moral vice, when I think she meant it as pursuing your own happiness, not necessarily at the expense of others. Any classical liberal or competent economist can see that when we act in our own self-interest - which in a society necessarily involves voluntary exchange and competition - it makes everyone better off. I think that's the reason she saw selfishness as a virtue, and I agree with her.