Because Bella said she wouldn't change for a while, at least not before her honeymoon. Jacob said that it was not a normal honeymoon anyway. Bella said it was actually no different from a normal honeymoon.

So Jacob knew that Edward would be with Bella when she was still human ... (That's it), so he expressed his anger and felt sick. He thinks Edward will kill Bella. You know, vampires are so powerful that they can crush Bella at will.