Bob identifies himself with a socially-constructed group. A nationality, a religion, a subculture, "Nice Guys", or whatever. As such Bob feels Alice is obligated to be with him, one way or another. Perhaps they identify with the same group, and she will be a 'traitor' to their category if she does not date or marry within it. Or maybe he just feels that by belonging to the category he 'deserves' her more than someone that does not. The former self-justification often goes hand-in-hand with the latter. If Alice complies, she might even discover that Bob is willing to take his sense of entitlement further still.