New roommate's pit-bull keeps growling at me. Why?

This is huge, wait until the owner comes home, then tell her that you need to interact with her and the dog. The dog needs to see you interacting with the owner with a good report. The dog needs to see you as a friend, and the only way that will happen is with the owner present. Do not try to establish dominance, the dog will only perceive this as a threatening act.