Two organism live in parasitic relationship when one gets benefits over the other.
In the case of flea and dogs, flea is benefited while dog is harmed. Flea is a blood sucking organism that prefers to live on/within the body of war blooded animals to get its nutrients. When flea lives on/within the body of dogs, it sucks the blood of dog thereby harming them iand hence flea and dog live in a parasitic relationship.
