Really? Cheerful? The false accusations of infidelity? The father wishing his only child was dead? The nearly averted duel between two friends? Cheerful?
One of the things that interests me about the play is how it constantly borrows the characters, plot devices, and even language of tragedy and turns them into comedy. Don John, for instance, seems a forerunner of Iago and Edmund in his motiveless villainy. The friar's idea of pretending a young woman is dead comes right out of Romeo and Juliet. Over and over again, the devices of tragedy are used for comedy.
This is seen in the Prince's line (much played upon in the piece) where he says: "In time the savage bull doth bear the yoke." The phrase became proverbial, but originally it came from a poem by Thomas Watson. The first few lines (themselves including a rough translation from a sonnet by Seraphine) run like this:
In time the Bull is brought to wear the yoke;
In time all haggard Hawks will stoop the Lures;
In time small wedge will cleave the sturdiest Oak;
In time the Marble wears with weakest showers:
More fierce is my sweet love, more hard withal,
Than Beast, or Bird, than Tree or Stony wall.
Shakespeare likely didn't get the line from Watson, though. He didn't need to. It was already much more famous from Thomas Kyd's use of it in The Spanish Tragedy. The villain Lorenzo says at the beginning of Act II:
My lord, though Bel-imperia seem thus coy,
Let reason hold you in your wonted joy:
In time the savage bull sustains the yoke,
In time all haggard hawks will stoop to lure,
In time small wedges cleave the hardest oak,
In time the flint is pierced with softest shower;
And she in time will fall from her disdain,
And rue the sufferance of your friendly pain.
Basically, he's telling Balthazar, son of the Viceroy of Portugal, that if the lady Bel-imperia is not moved by friendly courtship, they can force her to do their will. For Shakespeare's audience, the Prince's lines might have carried some dark overtones, and they almost certainly would have reminded people of Kyd's bloody play.
The villains in The Spanish Tragedy ultimately make good on their threats and kill Bel-imperia's lover, Horatio. The murder scene, memorialized on the frontispiece for the published edition of the play, ends like this:
BEL-IMPERIA: O, save his life, and let me die for him!
O, save him, brother! save him, Balthazar!
I loved Horatio, but he loved not me.
BALTHAZAR: But Balthazar loves Bel-imperia.
LORENZO: Although his life were still ambitious, proud,
Yet is he at the highest now he is dead.
BEL-IMPERIA: Murder! murder! help! Hieronimo, help!
LORENZO: Come, stop her mouth! away with her!
Here's the image that appeared in the published script:
Notice the villain on the right is speaking the famous last line "stop her mouth!" Does that line sound familiar? It should.
At the end of Much Ado About Nothing, a character says of Beatrice: "Peace! I will stop your mouth." Now the character attributions in the play are notoriously problematic. (At one point, Dogberry is referred to as Kemp, the name of the actor who played him.) Here, the original text lists Leonato as the speaker, but most editors agree that the line belongs to Benedick, and the traditional stage direction there is that he kisses her.
So what is Shakespeare doing? He's taking a line famous for its use in tragedy and reworking it into a confirmation of romantic happiness that resolves a comedy. Gutsy, innovative, but hardly cheerful from beginning to end.