At this point in the poem, More has begun to cry. John Donne is trying to comfort More by telling her not to cry or sigh because it would "cause" floods and storms that would only make his journey more difficult. This is showing the idea of a Petrarchan woman. A Petrarchan women is a lady who is able to control the power of her lover through her emotions. He is saying that More's love for him is so powerful that it can actually cause physical disasters , so if she begins to grieve his death it will actually cause his death. This is also an example of a hyperbole.