You make a good point, and if I think about it, this is also true of me. The difference I can think of, is that once I let someone in, they really have to do something awful, or let me down consistently over a long period of time for me to shut them out again. The ENFJs I know of don't give so many chances, or simply know the person will let them down before it happens, so don't befriend them in the first place.