Here are the questions you have to ask your self. Is the one your in the relationship with connect with you enough? Does your BF do things that you would not tolerate from a partner if so best to stay friends. If you answered no to the connection question ask yourself if it might is because of you or them. If it's you then examine why you keep this person at arms length. If it's them ask them why they keep you at arms length. Often time our commitment fears come out when we don't feel engaged or connected enough with our partners. Lastly if you choose to pursue the friend then always remember that once you cross that line the friendship will forever be connected to the relationship and if it goes sour then then friendship is almost always lost as well.