Answer: Juliet is remarking on how quickly Romeo seems to be advancing in their relationship.
Context:
The tale of the star-crossed lovers: Romeo and Juliet, involves a forbidden love between the members of the Capulet (Juliet) and the Montague (Romeo) family. Aware that their love involves consequences that neither wish to behold, they keep it fairly secretive, with each advance of their relationship having the potential to jeopardize each other with an even greater magnitude.
Interpretation:
When Juliet says "I have no joy of this contract tonight." She is essentially saying that while she is in love with Romeo, she does not feel bound or in her words binded to a contract that is their love. I.e. Marriage, wedlock. Such a grand decision can not always be dealt with in a short, timely manner.