Note to Twelfth Night, 1.5.188-189, "what is yours to bestow is not yours to reserve"

Cesario/Viola means that Olivia, a lovely woman, has the right to give her love to the man of her choice, but not the right to deny love altogether. Therefore she really does "usurp" (wrongly take the place of) herself, because she is made to love and be loved, but she has dedicated herself to mourning her brother's death. The general idea that love is natural, life-giving, and not to be denied is one that appears repeatedly in Shakespeare's comedies.

Imogen Stubbs as Cesario/Viola, left and Helena Bonham Carter as Olivia, right—1996