How do the events in Act III, Scene 1 establish Romeo as "Fortune's fool"?

In Act I, Scene 4, Romeo admits his life is guided by fate. He fears that this night, as he attends the Capulet party, will be the beginning of the end of his life. He says,

I fear too early, for my mind misgives
Some consequence yet hanging in the stars
Shall bitterly begin his fearful date
With this night’s revels, and expire the term
Of a despisèd life closed in my breast
By some vile forfeit of untimely death.

Romeo's destiny leads him to Juliet and they fall madly in love, setting the stage for the tragic events to come. Even in Act II, Scene 6 when he is about to marry Juliet, he says he is ready to die once he is joined with the girl:

Do thou but close our hands with holy words,
Then love-devouring death do what he dare,
It is enough I may but call her mine.

The events in Act III, Scene 1 seal Romeo's fate and establish him as "Fortune's fool" (line 142). He unsuccessfully attempts to dispel the anger of Tybalt. When Romeo backs down, Mercutio views it as an insult and fights Tybalt, winding up cursing the families (foreshadowing their final despair) before dropping dead. Romeo steps in, and even though he knows it may change his life or even result in death, he engages Tybalt and kills him. This scene is considered the turning point in the play. It leads inevitably to the double suicide in the final scene. Romeo becomes the ally of fate as he hastens the complications which will result in his banishment, the proposed marriage of Juliet and Count Paris, and Friar Laurence's scheme for Juliet to fake her death.