An "enabler" is a person who advises, encourages, or facilitates potentially negative or self-destructive behavior in another person.

The Nurse and Friar Laurence are enablers. They're unwitting and somewhat inept enablers at times, but they're enablers nonetheless.

The Nurse raised Juliet since she was a baby. She's a loving and caring surrogate mother to Juliet, whose own mother is cold, distant, and businesslike towards her. Juliet's mother is incapable of simply talking with Juliet and relies on the Nurse for everything to do with Juliet except for an occasional unfeeling reprimand.

The Nurse happily serves as the go-between in Romeo and Juliet's brief, ill-fated courtship by taking messages back and forth between them. She also encourages and helps to facilitate their marriage.

The Nurse doesn't understand Juliet's intense, romantic kind of love for Romeo, but she nevertheless supports Juliet's love for Romeo, at least until Romeo kills Tybalt.

This seems to be the turning point in Juliet's relationship with the Nurse. With Romeo disgraced and banished from Verona, the Nurse encourages Juliet to forget about Romeo, marry Paris—as Juliet's parents and societal conventions dictate—and get on with her life.

Juliet feels betrayed by the Nurse for not supporting her marriage to Romeo. Juliet still loves her and needs her for emotional support, but she no longer trusts her or confides in her. Juliet tells the Nurse nothing about her plan to take a potion that will make her appear dead, until Romeo can come back to Verona from Mantua and rescue her from her tomb.

The Nurse's motivation for what she does seems to be entirely focused on her love for Juliet. She simply wants Juliet to be happy, fulfilled, and, above all, married.

Friar Laurence cares about Romeo and Juliet, too, but he has an ulterior motive for encouraging Romeo and Juliet's marriage. Friar Laurence believes that the marriage will somehow bring about an end to the "ancient grudge," the never-ending feud, between the households of the Montagues and the Capulets.

ROMEO. Then plainly know my heart's dear love is set
On the fair daughter of rich Capulet;
As mine on hers, so hers is set on mine,
And all combin'd, save what thou must combine
By holy marriage....

FRIAR LAURENCE. ... But come, young waverer, come go with me.
In one respect I'll thy assistant be;
For this alliance may so happy prove
To turn your households’ rancour to pure love. (2.3.58-62, 92-95)

To fulfill his goal of ending the feud, Friar Laurence secretly marries the couple, hides Romeo in his cell after Romeo kills Tybalt, lectures Romeo at length against suicide, rationalizes Romeo's killing of Tybalt, and, along with the Nurse, encourages Romeo—who has been banished from Verona on pain of death—to return to Verona and to the Capulet's own home, of all places, to spend his marriage night with Juliet.

As for Juliet, Friar Laurence lies to Paris about his marriage to Juliet that Friar Laurence knows will never take place, asks Juliet if she'd rather kill herself than marry Paris—she says she would—and gives her a potion which Friar Laurence just happens to have close at hand that will cause Juliet's body to counterfeit death until such a time as Romeo can return from Mantua, rescue Juliet from the Capulets' tomb, and take her away back to Mantua, where they can live happily ever after.

In the meantime, Friar Laurence sends Friar John with a letter to Romeo in Mantua explaining all of this sleeping potion business.

What could possibly go wrong with this scenario? In a word, everything.

Romeo never gets the letter from Friar Laurence because Friar John is involuntarily quarantined in a house in Verona because of an unnamed "pestilence" (5.2.10) that's mentioned nowhere else in the play.

Romeo arrives at the tomb too early, Juliet wakes up too late, and Romeo and Juliet kill themselves.

Romeo kills himself because he arrives in the tomb too early—even after taking the time to kill Paris outside the tomb and lay him in the tomb—and because he thinks that Juliet is dead. Juliet kills herself because she awakes from the potion too late and finds Romeo dead—actually dead, not dead from a potion like Juliet was. But Romeo wouldn't have been actually dead if Juliet would have awakened from the potion sooner or if Romeo would have taken a little bit longer to kill Paris and arrived in the tomb after Juliet awakened from the potion.

All in all, Friar Laurence's scheme was a magnificent tragic failure that resulted in the deaths of Romeo, Juliet, and Paris. Paris had nothing to do with any part of Friar Laurence's scheme, but was at the tomb simply to pay his respects to Juliet.

Ultimately, the marriage of Romeo and Juliet does bring an end to the feud between the Capulets and the Montagues, but probably not in the way that Friar Laurence hoped it would.

The Nurse does not act as a mentor to Juliet. She is a live-in servant to the Capulet family and has cared for the teenager all her life. This devoted, sentimental woman is very concerned with her young mistress’s welfare and happiness. The Nurse follows Juliet’s instructions when possible and even helps the lovers get together. However, she understands that the parents have their daughter’s best interests at heart and that her own status is dependent on the adults. As she is attached to the household and is of a much lower social class, her ability to mentor Juliet is severely constrained.

Friar Laurence is also somewhat limited in his capacity as a mentor because he is a monk, not a married man, and so lives somewhat outside society. He serves as a confidant to both young people and tries to serve as the voice of reason. He seems to lack moral resolve, however, as he ultimately gives into aiding them with in a bizarre plot. His vacillation, unfortunately, contributes to the young people’s tragic ends.

The Nurse and Friar Laurence play similar key roles in Romeo and Juliet considering they both act as advisory parties and surrogate parents. Since the Capulets and Montagues are so hopelessly hateful towards each other, Romeo and Juliet have no other choice than to confide their love for each other with the Nurse and Friar Laurence. 

Friar Laurence gives both Romeo and Juliet advice throughout the play. He urges them to exercise more caution with their reckless love, especially when he reminds Romeo of how quickly he forgot about being in love with Rosaline. However, his advice falls on deaf ears and he eventually agrees to marry the two of them. His intentions, while they appear rooted in love, are not as clear as they seem, since we learn he is attempting to mend the rift between the two families with the marriage. One could argue that he goes against his own advice of caution by underestimating the damage Romeo and Juliet's marriage could cause. And speaking of damage, he plays a part in the ultimate demise of both characters, begging the question: couldn't he have helped avoid all this instead of helping cause it? 

The Nurse is more closely tied to Juliet, as she serves as a maid in the Capulet household and a second mother to Juliet. She mostly serves as a messenger between the two lovers. However, when Romeo is banished she is quick to urge Juliet to forget him and marry Paris. This shows that she has Juliet's interests at heart, even if Juliet only wants Romeo. The Nurse is also bound by societal expectations considering her position in the Capulet household, and isn't likely to tell Juliet to run away with Romeo. 

Both the Nurse and Friar Laurence mean well and want to see Romeo and Juliet happy, whether that means being together or not. Both do what they can to bring the two together in marriage, but when things start to fall apart, pressure gets the best of them and they both retreat. Unlike Juliet and Romeo, they aren't willing to sacrifice everything for love. Whether all the decisions they made were "right" is debatable, but they always had good intentions and cared for Romeo and Juliet, which is what matters the most.