Two elements that build up tension are the escalations of the conflicts as well as the occurrence of the climax.
While Paris's love interest in Juliet has always loomed as a problem, it's not until we get closer to the climax that it becomes a major point of conflict. Up until the middle of the play, the greatest conflict separating Juliet from Romeo is the feud between the two families, which can be described as a character vs. character conflict, as well as the fact that both Juliet and Romeo were fated to be born into the two warring families, which can be classified as a character vs. fate conflict. However, after Tybalt's death, Lord Capulet sees Juliet's grief as being so severe and unhealthy that he feels distracting her with marriage is best. So, suddenly Juliet is in a terrible conflict with both her parents as well as her circumstances. If she gives into her parents' will, she becomes guilty of the atrocious sin of polygamy. Friar Laurence's solution to the conflicts of faking Julite's death leads us even closer to our climax, creating even more tension.
The climax is not only the most emotionally intense moment of the play, but also the moment when the resolution becomes inevitable. While the moment that Juliet is faced with the decisions of either having to marry Paris, commit suicide, or take Friar Laurence's potion is a tense moment, the true climax would revolve around Romeo as he is the play's tragic hero. The true moment of climax is when Romeo is informed by his man servant Balthasar that Juliet has died. Sadly, Friar Laurence's letter has not reached Romeo in time, and he believes her death is real. This is the moment when Romeo decides to take his own life in his hands, the moment when the play's resolution becomes inevitable, the resolution being both his and Juliet's deaths. We especially see Romeo make the decision to take his own life when, after Balthasar tells him the news of Juliet's death, Romeo proclaims, "Is it e'en so? Then I defy you, stars!" (V.i.24). In saying this, Romeo is saying that he is challenging fate. While it seems that fate has set out for him a path of grieving and suffering, he is saying he refuses to succumb to that fate, and is challenging fate by taking his own life into his hands. Romeo's decision to take his own life before he learns all of the facts concerning Juliet's apparent death certainly adds to the tension of the play, leading up to the final tragedy.