"Wherefore" does not mean "where?" but "why?" Juliet is asking, "Why must you be Romeo Montague?" She knows he belongs to the family that has been feuding with her own family, the Capulets, for generations. She can't understand why a little thing like a family name should create such a problem for her and the young man she so passionately loves. Being a female, her family name is not important to her. She would gladly change her name to Montague and become part of Romeo's family if it were possible. That is why she says:
O Romeo, Romeo! wherefore art thou Romeo?
Deny thy father and refuse thy name!
Or, if thou wilt not, be but sworn my love,
And I'll no longer be a Capulet.
Juliet is a very young girl and has lived a sheltered life. There is a lot she does not understand. It is, of course, customary for a woman to take her husband's family name when she marries, but a man's family name is more important to him, more a part of his identity and self-image. Romeo could not change his name as readily as Juliet, even though he loves her just as much. Her little soliloquy about names is evidently to show her youth and naivete. It also shows Romeo that he can have her as his bride if he wants her and is willing to accept the problems that would go with a marriage between members of two rival families.
This scene is crucial to the play and the one most people remember best. The essence of the plot is that two young people fall in love regardless of the fact that their parents hate each other. This basic premise has been copied many times over the years, perhaps most successfully in an old play titled Abie's Irish Rose, in which a boy from an Orthodox Jewish family marries a girl from an Irish-Catholic family and creates an uproar between their respective parents. According to Wikipedia:
Although initially receiving poor reviews, the Broadway play was a commercial hit, running for 2,327 performances between May 23, 1922 and October 1, 1927, at the time the longest run in Broadway theater history.
This question has already been answered. Here is a link for you: http://www.enotes.com/romeo-and-juliet/q-and-a/o-romeo-romeo-wherefore-art-thou-romeo-1217
It basically just means in modern words , " Romeo where are you ? "
Basically what she is saying is that she wishes Romeo was not a Montague, and that she longs to be with him but is sad that he is part of her families rival family.
in other words she is saying "Where are you Romeo?"
In this scenes Juliet is longing for her beloved Romeo who has so tragically left her. If you read the script, or even watch the film you will understand this.
Juliet is wishing Romeo to be with her: in modern languge "Romeo, Romeo where are you Romeo..."
In Juliet's monologue she is asking where is her Romeo. The repetition of Romeo's name highlights her longing for him.
This question has been previously answered: