In Jane Eyre, why does Jane decline St. John's offer to travel with him to India?

Jane declines St. John's offer because she is not in love with him and doesn't want to marry him.

Originally, she says that she will travel with him to be a missionary, but when he continually pressures her to marry him, she refuses. Her decision is final when she believes that she hears the voice of Rochester, and, taking it for a sign, she firmly says she will not travel.