One of the most effective aspects of the ending to Romeo and Juliet is the fact that Lords Capulet and Montague finally unite. This aspect of the ending actually changes the play from being a traditional tragedy to a much less traditional tragedy. In fact, scholars classify Romeo and Juliet as one of Shakespeare's Problem Plays.
The unity of Lords Capulet and Montague is first lead by Prince Escalus's very poignant and effective speech pointing out that God has seen fit to punish Lords Capulet and Montague for their hatred by uniting the two families through the love between their children and then killing their children. We see Prince Escalus making this effective declaration in the lines:
Where be these enemies? Capulet Montage [Montague],
See what a scourge is laid upon your hate,
That heaven finds means to kill your joys with love! (V.iii.302-305)
Prince Escalus creates further unity in this speech by pointing out that God is not only punishing them but punishing everyone. God has especially punished Prince Escalus by killing some of his family members, such as Mercutio, for not forcefully putting an end to the feud sooner, as we see in his lines, "And I, for winking at you, discords too, / Have lost a brace of kinsmen. All are punish'd" (305-304). The phrase "winking at you" can be translated as turning a blind eye, showing us that Prince Escalus is guilty of having ignored the feud for a long time. The line that most passionately creates unity in this speech is Prince Escalus's final proclamation, "All are punish'd," showing us that all people present have a share of responsibility in all of the deaths that have taken place.
Prince Escalus's speech that serves to unify all people through universal blame for the deaths inspires Lord Capulet to call Lord Montague "brother," as we see in the line, "O brother Montague, give me thy hand" (307). The term "brother" serves to remind us that they are now related by marriage but also to remind us that they are brothers in a spiritual sense. Montague equally repents and accepts Capulet's offer of brotherhood, even offering to raise a statue of "pure gold" in Juliet's honor. Capulet responds in kind and offers to raise a statue in Romeo's honor, creating permanent forgiveness and unity between the two families. It is this unity, this resolution, that makes the ending of the play so effective.
My subscription to enotes has run out. Absent from other student guides is Isaac Asimov's note that the so called potion the Friar gives Juliet is a fictional matter. The effects can't happen. So, we might pardon Romeo for killing himself. Someone asked about the County Paris character and it seems that at the end he is like Romeo was at the beginning; consorting with "the humorous night." Paris also attempts to do what Romeo had done, that is, taking the law into his own hands, or law enforcement.
Probably the fact that the tragic ending could have been easily avoided has something to do with it. It is ironical that they almost got away with it (that is managed to trick their family and live happily ever after).
If Romeo only wanted a bit longer, Julia would have awoken and the tragic ending of their young lives (despite all the hardship they had to endure) would have been avoided. However, it shows just how ironic can life be. It is desperately sad to think that they have gone trough so much and nevertheless lost their lives in the end.
Additionally, the fact that they both refused to go on without their love one gives their relationship dept. Romeo kills himself thinking that Julia is dead, and she does the same. It says something about their passion for one another and again makes the ending even more tragic and memorable.