Why would I not believe you? [after all, it's not as if cassius should judge.]

But I know I was not on the verge of death before this dream, so I doubt you're headed to death. [it doesn't occur to him that perhaps the living and the dead can meet here, but there are many illogical things that cassius does not take into account simply because they're illogical.] Perhaps you have a different reward waiting for you.