How does Shakespeare explore the theme of fate in Romeo and Juliet?

Shakespeare's Romeo and Juliet opens with a prologue that details the subject of fate. In a description of the events to come, it is stated that:

From forth the fatal loins of these two foes

A pair of star-cross'd lovers take their life;

Whose misadventures piteous overthrows

Do with their death bury their parents' strife.

The particular use of the term "star-cross'd" suggests that Romeo and Juliet's relationship was doomed from the start—born, bred, and eventually killed off by the poor alignment of the stars or dismissive hand of fate. Is this true, though? Is fate responsible for the tragedy that occurs in this play, or are characters' impulsive actions and mistakes the source of their downfall? 

Shakespeare never gives us a concrete answer on the matter, but he certainly does reference fate frequently throughout the play. Prior to the Capulets' ball, Romeo states:

...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 despised life closed in my breast

By some vile forget of untimely death.

But He, that hath the steerage of my course,

Direct my sail! On, lusty gentleman.

Here, Romeo is predicting that the night ahead of them will be fateful, but not necessarily in a good way... rather, in a manner that ultimately will be resolved in death. Regardless, Romeo embraces his "fate," and fateful the night is; Romeo meets Juliet at this ball, which sets off the events of the remainder of the play.

Other references to fate are aplenty within the text. The metaphor of stars as guiding entities re-enters when Romeo describes Juliet's eyes as "two of the fairest stars in all the heaven." Within Juliet's eyes lie Romeo's fate. When Mercutio dies in a duel with Tybalt, Romeo exclaims, "This day's black fate on more days doth depend," which implies that, once again, another death was determined by fate. When Romeo falsely learns of Juliet's "death," he screams out, "I defy you, stars!"

Overall, Shakespeare explores the human tension between taking accountability for our actions and wanting to ascribe our problems to fate. While the characters within Romeo and Juliet often look to fate or the stars to deal with the losses they experience, they ultimately have to suffer the consequences of their actions on this earthly plane.