In the beginning, even when you really like someone, but you are simply enjoying the flow of things, the addiction hasn't kicked in yet. The thought process might be "you know what, if you decide to be with someone else tonight, go ahead; it'll be difficult to reach the level of gentle and effortless devotion and chemistry that we've generated so naturally; so, be free, and I will be free". The moment there is a formal, explicit arrangement, it seems like the crack aspect starts to take hold.