Of course if Romeo and Juliet were real people instead of characters in a drama, you might be able to make the argument that they could have made different decisions. For those of us in the audience, there are certainly ways we could see for them to avoid the tragic end to their tale. If Romeo had just waited and dragged out his speech a little longer, Juliet might have woken up in time. If they had just cooled their heels for a bit and lobbied their parents, there is always a chance they would relent.
But according to the play, and the other things the play was based on, these guys were doomed from the start, they were "star crossed lovers" and their fates were determined and set in stone if you will.
Because so much of The Tragedy of Romeo and Juliet refers to fate, I think Shakespeare intended for it to end in this tragedy somehow. From the Prologue's "star-crossed lovers" to Romeo's fear of his early death in the words:
for my mind misgives
Some consequence yet hanging in the stars
Shall bitterly begin his fearful date (I.v.)
to Romeo's "I defy you stars!" after killing Tybalt, we see this commitment to the power of fate. In each situation, it presents the circumstances of life as out of the control of mankind's ability to make decisions that will affect their circumstances.
Sure, if they had made different decisions, things may have turned out differently, but according to the seeming belief in the power of astrology, these two were bound to be troubled somehow.
No, the fates of Romeo and Juliet were definitely written in the stars. Through out the story Shakespeare constantly hinted at the fates. For example close to the end the Friar talks about a "Higher Power" this being Fate and how no person has control over it. Through fate, the Frair does not make it to Juliet's tomb on time. Romeo kills himself before the friar can tell him that Juliet is not really dead. This is not the friar's fault. Rather, it is fate that he did not get there on time .This example from Shakespeare should be easily perceived of that it was indeed fate that has taken over the tragic and harsh death of Romeo and Juliet. Even if they had made different decisions and choices, the outcome would still have resulted in their deaths. These beliefs, foreshadows, reminders, and signs of probable occurrences being written in the stars explain how life is predetermined by fate. It is something that one cannot control, and hence, it is the reason that the two young "star-crossed" lovers in this fantastic tragedy encounter what is fated to occur.
Hope this helps =)