You have to start right at the beginning to be friendly and try to chat them up. Accept only answers where he/she is somewhat friendly. If the answer is negative, just go back (effectively undoing the action) and try again after you did something else. The probability is not that low to get a positive answer. If you consistently chat with them and abuse the back option so you build a positive rapport, once the conflict with her boyfriend/his girlfriend occurs it should be sufficient to stay in the positive zone. Be consistent and do the friendly chat all the time and slowly but surely your roommate won't hate you. That's at least how it works in my case.