"Fate" or "Destiny", you could say, appear strongly throughout the play. Yet character influences feature just as much.
The idea of fate is a power that is believed to control events, nothing can stand in its way, whatever it is, and it's bound to happen under any circumstances.
Romeo and Juliet would have died whatever happened - Inevitable. Not forgetting the fact that certain characters play significant parts in the consequences of events.
The Elizabethan times people strongly believed in the idea of fate/fortune, they believed that there is not a lot you can do about it, if fate is involved there is nothing you can do about it. While also believing that you have good and bad luck, and there is nothing you can do to alter proceedings, good or bad you just had to accept what happens to you.
We get a strong feeling of fate straight off from the prologue, already with the knowledge that it is talking about Romeo and Juliet, we get the idea that there will be deaths around them and, obviously to them, in various circumstances. As shown here:.
"The fearful passage of their death-marked love".
It also suggests the way of death, and that their fate is decided.
"Which, but their children's end, nought could remove".
This, along with the first line: are the first mentions of the two families This sense of fate also seems to show that key events that occur in the play are planned regardless of what the characters say or do.
"Doth with their death bury their parents strife".
Exploring that Romeo and Juliet's deaths end the conflict between the two households.
"And the continuance of their parent's rage" .
Up until their deaths the conflict between the two family's remained, unaware of happenings around them.
At the beginning of the play Romeo and Juliet are described as "Star crossed lovers".