It is important to realise that fate, fortune and stars are words that appear many times in this excellent tragedy and relate to the sense in which destiny can often decree a path for our lives that we cannot escape. From the very beginning of this play, in the Prologue, we are told that Romeo and Juliet are "star-crossed lovers." Both Romeo and Juliet have strange premonitions of the fate that is to befall them at various points in the play, such as Act I scene 4 for Romeo. However, it is only in Act III scene 1, after the chain of events has been unleashed that has seen Mercutio killed by Tybalt, and then Romeo forced to challenge Tybalt for the death of his friend, even though they are now related by marriage, and then his killing of Tybalt, resulting in his exile, that Romeo fully appreciates how the hand of fate, or fortune, is working against him. He at this moment recognises the implacable forces that are set against a "happy ending" for him and Juliet.
His tortured ejaculation, where he identifies himself as "fortune's fool," is thus an example of foreshadowing, as well as being the climax of this scene. He begins to have some sense of the inexorable forces that range themselves against him and Juliet, and recognises that this can only end in one way: death.