Like a lot of Shakespeare plays, The Two Gentlemen of Verona toys with the common sixteenth-century notion that love can transform men and women (but mostly men) into something unrecognizable. Ultimately, however, such transformations reveal more about characters' identities than the transformations conceal. The clearest example of this is when the disclosure of Julia's "Sebastian" disguise prompts Proteus to discover something about himself as he declares "O heaven! were man/ But constant, he were perfect" (5.4.7). The point seems to be that, while human beings can be fickle, changeable, and unstable, they are also capable of some pretty important self-revelations.
Romantic love has the capacity to transform men and women into monsters.
Julia's transformation from "Sebastian" back into "Julia" is the catalyst for Proteus's realization that his changeable nature is his worst a flaw.