In Shakespeare’s time all it took for a couple to marry was for them to swear an oath of their love for each other; there was no requirement for a justice of the peace or for the signing of any documents. Just saying “I swear I love you” was all it took, and you could immediately consummate the marriage without committing a sin or breaking the law. For a marriage contract to be enforceable in a court of law, however, required a witness — but that could be anyone, even the barmaid in a pub. So in this scene, when Romeo swears his love for Juliet, he’s essentially proposing that they become married right here and now. It’s no wonder that Juliet’s not happy rushing into such a contract, especially without a witness.