If lying was a necessity for Juliet in Romeo and Juliet, how was it also a bad thing?
Well, Juliet was in a really difficult situation. She was in love (or thought she was in love) with a guy who her parents would NEVER let her marry. In Juliet's mind, it was necessary for her to hide her marriage and lie to her parents in order to be with Romeo. But maybe the story would have turned out differently, less tragically, if Juliet hadn't lied.
One of the most fun things about Romeo and Juliet is asking and trying to answer "what if" questions. What would have happened if the star-crossed couple had run away together after getting married? What would have happened if Juliet hadn't refused to marry Paris, thereby angering her father? What would have happened if Romeo and Juliet had not rushed into marriage? After all, they only met each other once; what if they had said, "Let's give this relationship some time?" What if Romeo hadn't killed Tybalt, or if Tybalt hadn't killed Mercutio? What if Juliet hadn't gone along with Friar Lawrence's plan, and instead gone off to Mantua to find Romeo? What your question is asking is, what would have happened if Juliet had informed her parents of her love for Romeo before getting married? Or after? All of the above seem like better decisions than the ones the couple made.
The essence of the story is that Romeo and Juliet were so rash in their love and their marriage (completely ignoring Friar Lawrence's warning, "Wisely and slow; they stumble that run fast") that their lives couldn’t go on normally. One problem piled onto another problem until they were dead. On the other hand, all the decisions they made, including Juliet's decision to lie to her family, were determined by fate, according to Romeo: "Oh, I am fortune's fool!" So no matter what your answers are to the questions above, the conclusion is always, "...but they had to because of fate." Also because the story would not have been very interesting if Romeo and Juliet had made better decisions.
It is often a necessity for Shakespeare's lovers to resort to lying or deception in order to be together. In Shakespeare's comedy A Midsummer Night's Dream, Lysander and his lover Hermia secretly run away together during the night, in spite of her father's disapproval. Lysander says, "The course of true love never did run smooth."
Romeo and Juliet is a perfect example of similarly "star-cross'd lovers," as the prologue says. The holy Friar Lawrence even participates in the deception by giving Juliet a sleeping potion. Romeo and Juliet lie for love, and the friar lies to unite their two families. The three have good motives in deceiving others. Also, even though the lies lead to the lovers' deaths, the warring Capulets and Montagues make “a glooming peace” and stop their senseless fighting.
However, lying also causes a lot of problems in the play. Juliet’s parents, nurse, and fiancé Paris are heartbroken, first over her fake death and then a second time when she actually dies. Juliet pretends to die so she can be with Romeo, but Romeo is also taken in by this lie before he can be contacted. At this news, Romeo commits suicide, and Juliet follows.
The reason why lying in Romeo and Juliet seems both necessary and bad is because of Shakespeare’s ability to create ambiguity. Ambiguity is a kind of uncertainty that can make fiction engrossing because it doesn’t give easy answers, just as we don’t usually have easy answers in real life. For example, Juliet’s desperate love for Romeo is understandable, but her nurse’s practical advice to make the best of her situation and marry Paris instead is also reasonable. It isn't easy to determine who's right. Thus, the lies told in Romeo and Juliet can be both well-intended and harmful.