Because they fell in love. But Juliet is a good girl; she says to Romeo "If thy bent of love be honourable, thy purpose marriage, send me word tomorrow." She is not going to enter upon a clandestine affair--although she wants very much to go to bed with Romeo, she will do it only when he is her husband.
Romeo and Juliet are in a hurry to be married because they are deeply in love and feel a sense of urgency due to the conflict between their families. They believe that getting married is the only way to be together despite the feud. Additionally, they are young and impulsive, making them willing to take risks for their love.
Romeo and Juliet meet on Sunday and marry on Monday. Clearly this is an absurdly short time, and it is unlikely that any real marriages even back then were arranged so quickly. But it suits the breathless pace of the play, and is plausible since these two are pretty eager to get into each others' pants. If you don't believe that, check out Juliet's soliloquy "Gallop apace, you fiery-footed steeds" in Act 3 Scene 2.
Another answerer said:
Romeo and Juliet married so early, because it was not considered early back when the story was written. Since the health of the people of the time was not great, and they only lived to about 40 back then, they wanted to get things done early. Teenagers that age were commonly married.
Actually, this is not true and does not answer the question. In Shakespeare's day, people normally married in their twenties, men and women both.
They married secretly because their families have been feuding and they do not want either family to notice they are married because both families are supposed to hate each other. : ) Hope i helped :P
They married in secret because both families hated each other dreadfully and they knew that if this came to public one of them would die. In this marriage they promised to love each other until the day they part. Which is when they die.
Juliet I think both. Romeo is in a hurry to get married to Juliet and asks Friar Laurence to marry them shortly after they've met. Juliet, shows she's impatient when she is waiting for the Nurse to come back and give her news about the plans to get married to Romeo
Romeo and Juliet get married.
Paris married Juliet for money in Romeo and Juliet.
Romeo and Juliet
Friar Laurence married off Romeo and Juliet.
Romeo, and Juliet.
After Romeo and Juliet married Romeo owned Juliet and everything she owed as well.
Friar Lawrence married Romeo and Juliet in his cell.
He was the priest that married Romeo and Juliet.
he got married in Italy
Juliet is 13; it's not clear how old Romeo is.
Juliet sends her Nurse to find out if she is to be married to Romeo.