If we think of the play as a "celebration" of male friendship, then we can also think of Proteus and Valentine's bromance as the play's major "protagonist." The relationship is put to the test by Proteus's disloyal behavior and the guys' romantic relationships with women. In the end, male friendship wins out when Valentine forgives Proteus for trying to steal his girl.
What? You don't like what we've argued so far? Fine. How about this – what if we argue that romance, not male friendship, is the play's "protagonist"? If we think of it this way, then it is male-female love that must overcome all obstacles. Why else, one could argue, would the play end in the promise of a double wedding?