One of the themes present in Romeo and Juliet is that fate shapes the characters' destinies regardless of how the characters themselves attempt to alter the eventual outcome. In the Prologue, the first whispers of the fates of Romeo and Juliet are found:
From forth the fatal loins of these two foes
A pair of star-crossed lovers take their life ...
Romeo and Juliet are "star-crossed," or ill-fated. This is a theme that runs throughout the play and is echoed again in act 3, scene 1.
In this scene, Romeo tries to avoid fighting Tybalt. He has just married Juliet privately, and he does not want to engage in conflict with her kinsman. Romeo tries to dispel the growing tension, but after Tybalt kills Mercutio, he is driven to revenge. In an emotional rage, Romeo fights Tybalt and kills him.
And that's quite an issue. Not only does he place himself into conflict with the Prince, but he also stands as an enemy of Juliet's family, and the two households were already on poor terms. After Tybalt falls dead, Romeo steps back from the situation and realizes the gravity of what has happened. Benvolio reminds Romeo that he'll be killed if he's caught, and Romeo delivers this line:
Oh, I am fortune’s fool!
Romeo believes that fortune, or fate, has played him for a fool. Although he tries to avoid conflict with Tybalt, the circumstances around him spiral out of control until his fate is sealed. This moment begins a series of "fateful" events that lead both Romeo and Juliet to their deaths.