Fleas and dogs have a symbiotic relationship.  In fact fleas can have this symbiotic relationship with any warm-blooded animal.  The relationship is a parasitic relationship. 

In a parasitic relationship one organism, the parasite, benefits to the detriment of another organism, the host.  The dog is a host, it provides a food source (blood) and shelter (the fur) to the parasite the flea. 

As I noted the flea can have this relationship with any warm-blooded animal.  Rats, birds, even humans.  It is a surprisingly common form of symbiosis.