Like several others, the origin of this phrase is also traced to Shakespeare. He has used this phrase in Act-II, Scene-II of his play, Romeo and Juliet. This scene takes place in the balcony, when Juliet says, “Sweet, so would I: / Yet I should kill thee with much cherishing./Good night, good night! Parting is such sweet sorrow.” Romeo and Juliet talked to each other the whole night and made a plan that Romeo would send her a message next morning about where they could get married. Here Juliet feels sadness and sorrow for saying goodbye, not knowing that what is coming their way would be beautiful, when they would see each other again.
Romeo and Juliet, being lovers, are desperately waiting to be together. This is the point when Juliet says this line to say goodbye to her lover, Romeo, anticipating another meeting the next day. In fact, she refers to the pain they had faced since their relationship started. It hurts her to leave Romeo. However, parting hurts her so much, and despite that it intensifies her feelings for him. She wishes to say him goodbye repeatedly until morning comes. Simply, you can understand the line that lovers hate to separate from each other when they are in love.
The phrase is very commonly used in advertisements and lovebirds often use it in their practical life. It may carry different meanings to different people. Some use it as a verbal wistful kiss at night, while others use it as an expression to their undying love, and still some others may use it to highlight the nature of their relationships. Parting is painful, because a beloved prefers to trap her lover in twisted chains. Fast friends can also use it jokingly when they part after finishing classes.
Juliet says this phrase in Act-II, Scene-II of Romeo and Juliet, when she parts from Romeo for short duration at night. This scene goes on as;
I would I were thy bird.
Sweet, so would I:
Yet I should kill thee with much cherishing.
Good night, good night! Parting is such sweet sorrow,
That I shall say good night till it be morrow.
Exit JULIET, above
(Act-II, Scene-II, Lines 188-189)
Juliet says this line to say goodnight to her lover, Romeo. This sorrowful parting gives them pleasure and looks “sweet” as it gives them hope to see each other next morning again.
This line manifest the inner pleasure and gladness of love that the lovers experience when they meet and part to meet again. As it is clear that, Juliet feels the moment of sadness and joy simultaneously, because love could transcend normal experiences and expectations such that her sense of sorrow turns into sweetness joy. To fall in love with someone is a gift, while to feel at parting is a sign of deep love that brings another meeting in prospects and hence joy. This phrase has become an iconic line of the play. For instance, to enjoy the taste of food, one must experience hunger and to be really happy, one must deeply sad. The same is the situation with Juliet, as saying Romeo goodbye triggers her deep emotions; nevertheless, sadness reminds her of their deep love and hence it is sweet. This means, saying goodbye initiates an anticipation of meeting him again.