What does it mean when you dream with someone?

There is not enough information to provide a meaningful response. If the question refers to dreaming about someone, it could mean any number of things depending on the identity of the "someone" as well as what happens in the dream. If the question refers to having the same dream as someone else, it generally means that the two dreams dealt with a similar subject, such as a black dog. Closer inspection almost always reveals many differences in the two dreams. In the example, one dream might have shown the black dog running through a park while the other dream pictured the dog tap dancing.