I think there are two things that control Romeo and Juliet. One is fate the other is freewill. Freewill and fate seem to disagree and conflict each other throughout the book. Romeo and Juliet choose to love each other and even though their families hate each other they fall in love and get married. This has very little to do with fate, but fate does interfere with there relationship. I think Shakespeare meant to use fate to have them die though.
One example of fate in their relationship is when Romeo before entering Capulet's house, he says (1.4.113) "I fear too early for my mind misgives some consequence yet hanging in the stars." He senses something bad is going to happen and he can feel it hanging in the stars (fate)
In 3.1.93 Mercutio Says "I Am hurt a plague o' both houses" These words come true. They may even have cause the tragedy of Romeo and Juliet. When they die this is a tragedy for both houses. When Mercutio says that he doesn't mean for his friend Romeo to die, he just wants something bad to happen to both houses He also didn't know that through this tragedy anything good will come out of it. This has to do with fate because it is fate that Mercutio died. It was because of his freewill he said "a plague o' both houses". What happens in this scene foreshadows the tragedy and the outcome of this "plague" In this scene both fate and freewill are at work. This shows that part of the tragedy is fate and the other part freewill.
Also in scene 3.1.142 there is another indication of fate and freewill. After Romeo kills Tybalt he says "O, I am fortunes fool. He says this after he realizes what he has done. He killed Tybalt out of madness he didn't mean to kill him. Maybe fate was interfering and with him while he was mad and made him kill Tybalt. This results in his banishment which leads to his and Juliet's deaths.
I think that Shakespeare purposely made fate part of his story. Romeo and Juliet were meant to bring the families together and make peace. Shakespear probably believed in fate and that's why it is such a big part of his story. Also like the fate, is freewill, this also plays a huge part in the story. Like the families fate and freewill were against each other throughout the book. When Romeo and Juliet die this brings fate and freewill together because they were both were the reason that Romeo and Juliet's death happened. The death had two outcomes, like a lot of other things in this book (Two People, Two families, two people died Tybalt and Mercutio, etc.) None of what happened could've happened if only one of these elements were at work,