"It is not so much the central characters of the play, Romeo and Juliet themselves, as the minor characters that are responsible for their tragic end." I agree with this analysis to a high extent, but have also considered the other possible reasons why they died. .
The most commonly seen reason for Romeo's and Juliet's downfall would be the feud between the Capulets and the Montagues. However, the over-looked, minor characters play an important role in the downfall of Romeo and Juliet. They were constantly pushing them into secrecy and forcing them to construct a large and complex plan that results in Romeo's and Juliet's deaths. .
Tybalt is the trigger that sends Romeo and Juliet off on their downward path. He is always causing trouble and never once in appears in the play without being in the battles. One can find him constantly harassing Romeo and trying to start a fight. When Romeo finally does fight him to get revenge, he ends up killing him and thus gets exiled as the Prince promised earlier in the play. Romeo getting exiled means that when Friar Laurence and Juliet plans their devious scheme, Romeo is not able to hear about it straight away. In fact, he never hears about it, so assumes Juliet is truly dead. .
Paris seems to keep everything Romeo and Juliet does very hasty as he wishes to marry Juliet in two days. This means that Juliet drinks the potion that night, where she speaks her monologue in her bed. If Paris wasn't going to marry her in two days time, then she would have waited for a reply letter from Romeo. None of the confusion would have arisen. Paris doesn't love Juliet, not as Romeo does, but instead his love is only skin deep. He never really gets to know Juliet. If he did, then she may have liked Paris better than Romeo, which would completely cancel out everything else. .
The Prince contributes by exiling Romeo near the beginning of the play. Romeo thinks this very unfair ("Tis torture and not mercy.