Despite their roles as adult counselors, neither Friar Lawrence nor the Nurse offer good counsel to the young people who seek their advice. Friar Lawrence does not serve Romeo well as a counselor when he agrees to marry the young couple. He is clearly thinking more about settling the feud than what is prudent for a young, lovestruck, and impetuous Romeo. He does not serve Juliet well as a counselor when he abets her in a plan to thwart her parents' authority, a plan that does not seem particularly consistent with his hope to end the feud. The plan is also physically risky, first regarding Juliet's life and second regarding a message getting to Romeo in a timely manner. The entire plan is fraught with possible problems.
The Nurse gives Juliet terrible counsel. First she assists her young charge in deceiving her parents without any real thought or deliberation or even discussion. Juliet wants Romeo so she helps Juliet get him. This romanticized behavior is understandable in Juliet but not in her adult counselor. Once Romeo has been banished, the Nurse gives Juliet equally bad advice--forget the marriage vows she made to Romeo because he is gone and make new ones with Paris because he is a fine man. She is just not reasonable and her counsel is neither mature nor consistent.
Lord Capulet and Friar Lawrence share the same weakness; that is they overstep their boundaries in their attempts to be problem solvers. These two seek hero status by trying to wield their influence to solve what they see as the greatest problem. The Friar does this when he marries Romeo and Juliet, thinking he will be able to act as the liason between the two rival families by explaining that now that the two are married, the family is one and the feud will have to stop. Of course, his plan goes astray and fails.
Lord Capulet can't stand to see his family so depressed after Tybalt's death, so he comes up with a not too well thought out plan to have his daughter marry Paris figuring it would take their minds off of their loss and focused on the joyous occasion of the wedding. Not only would such an action probably not heal the hurt the family felt, but it sends Juliet over the edge to the point she is willing to fake her own death and leave her family forever. This is another example of a tragedy caused by the overriding need to be a problem solver.
When we judge these characters, we do so from the perspective of a modern societal mindset. One of the flaws of Juliet, based upon the time of the play, is that she broke her family's legal commitment to Paris. This would have been inexcusable in most circumstances, but Lord Capulet explains to Paris, that he let Juliet make the choice. Of all the young men presented in her circle as acceptable Capulet thought Paris was the best, but Juliet also agreed to it.
The main characters would fit well into the twenty-first century as there is no deliberateness about them and little loyalty. Romeo does not consider the consequences of falling in love with the daughter of his enemy, Juliet forgets the danger in her passion for Romeo, and Friar Laurence thinks he is above the law with his actions regarding the marriage of the two youths and his plan for Juliet. In addition, the Nurse does not consider the consequences of advising Juliet to become a bigamist.
When the star-crossed lovers are in difficulty, the Nurse does not defend Juliet to her parents as they berate her;nor does the Friar aid Juliet after she awakens in the tomb; instead, he flees.
Lord and Lady Capulet don't listen to their daughter or give her credit for knowing her own mind. They arrange a marriage for her (which may have been "done" in that time) but then dismiss Juliet's complaints about the match. They are only concerned with what the family's have to gain from the marriage and for having control over Juliet.
Tybalt's weakness is the easiest of these. The man is a complete hothead with no real ability to control himself. He has an intense hatred for the Montagues and for Romeo in particular. This hatred and this inability to control himself is what leads to his death.