Romeo changes Juliet’s opinion about marriage. Before she meets him, she is not interested in marriage, but she agrees to marry him.
We do not know much about Juliet before she meets Romeo, but we do know that she was not interested in marrying. Juliet is almost fourteen, so it’s time she gets on with it. When she is asked what she thinks of marriage, she gives what can be interpreted as a very sarcastic answer.
It is an honour that I dream not of. (Act 1, Scene 3, p. 24)
Juliet’s nurse and mother discuss the concept of marriage. When they ask her about Paris, she eventually gives another answer, but it is just as evasive.
I'll look to like, if looking liking move;
But no more deep will I endart mine eye
Than your consent gives strength to make it fly. (Act 1, Scene 3, p. 25)
Apparently Paris is good to look at, and Juliet will defer to her parents on the matter.
Yet once Juliet meets Romeo, all this caution flies out the window. It is love at first sight.
My grave is like to be my wedding bed. (Act 1, Scene 5, p. 34)
Now Juliet is ready to go beyond looking. She wants to marry him, and no one else. When he asks her to marry him, she consents.