Why does Juliet react the way she does to Romeo's death in Romeo and Juliet?
Juliet kills herself when Romeo dies because she is caught up in the passion and fervor of young love, and cannot accept the alternative of a life with Paris.
Juliet fakes her death to get out of marrying Paris because from the moment she laid eyes on Romeo, she was sure he was the one for her.
Go ask his name.—If he be married,
My grave is like to be my wedding bed (Act 1, Scene 5, p. 34)
Juliet is smitten. Before she even meets Romeo, she falls in love with him. She goes from not really being interested in marriage to accepting his offer that night. Often when young people fall in love, that love burns bright with passion in the beginning of the relationship. Romeo and Juliet were still in that early stage.
Romeo kills Tybalt and is banished, and this is a real test of Juliet’s fortitude. She refuses to marry anyone if she cannot take Romeo. She will die alone.
But I, a maid, die maiden-widowed.(140)
Come, cords; come, Nurse. I'll to my wedding bed;
And death, not Romeo, take my maidenhead! (Act 3, Scene 2, p. 70)
Juliet is able to marry in secret, and she does so even though she knows Romeo is banished. This is no passing fancy. She is deeply in love with him, and willing to be with no one if she can’t have him.
When Juliet has her one night with Romeo, she is happy. However, she knows he has to leave when daylight comes, before her family finds him there. Romeo tries to tell her it is not the lark she hears, but she tells him it is. She admits that she wishes it were not.
O, now I would they had chang'd voices too,
Since arm from arm that voice doth us affray,
Hunting thee hence with hunt's-up to the day!
O, now be gone! More light and light it grows.(35) (Act 3, Scene 5, p. 78)
Juliet does not want to be separated from her new husband for anything. When she finds out that her parents have tried to set her up with Romeo, she tries to refuse. Her father threatens to disown her and set her out on the street, so Juliet goes to Friar Lawrence and asks for help. He arranges for her to fake her death.
Upon finding Juliet supposedly dead, Romeo kills himself. Friar Lawrence comes and sees Romeo dead and Juliet waking up, and tries to get her to leave but she refuses. She tells him she will not go, and tries to kiss his lips to get some of the poison.
I will kiss thy lips.
Haply some poison yet doth hang on them(170)
To make me die with a restorative. (Act 5, Scene 3, p. 111)
In the end, what Juliet does makes sense and fits in with her character. Everything she has done since meeting Romeo revolves around keeping him. Without him, she cannot bring herself to live. She prefers to follow him in death.
Romeo and Juliet are definitely young, and neither makes the best choices. Juliet is rash and intractable, and what she wants she wants. This is why she kills herself when Romeo dies. However, there might have more some more rational reasons. After all, she was married in secret. If she marries Paris, it will be a farce. Juliet has few choices left. She takes the way out that is available to her.
Juliet and Romeo's love is of such sudden intensity that their actions and emotions are larger than life, making for heightened dramatic effect in one of Shakespeare's most enduring stories. Upon seeing that Romeo has taken poison, and knowing he did this because he didn't want to live anymore because he thought Juliet was dead, she can think of no other solution but to kill herself, and when she sees there is no poison left, she stabs herself with his knife ("Oh happy dagger!"). The action of stabbing also lends a bit of phallic/sexual imagery to her death in a play full of sexual verbal imagery. These actions may seem extreme but the entire play is characterized by a sense of urgency and impatience, and the heightened emotions of other characters lead them to foolish and damaging behavior that have negative repercussions (as when Tybalt slays Mercutio, and Romeo slays Tybalt, leading to his banishment). The ongoing feud between the families is finally offered as the source of all this heartbreak and at the play's end there is regret and a decision to end the feud, in the wake of the tragic deaths of the two lovers.