Rejection can cover so many things. I've been thinking of its full implications ... that we are not loved, don't measure up, are not personally acceptable, don't have the attributes people want (and therefore won't fit in), we are going to be exiled from the group, etc. I suppose if we have a fear of any of those things being potentially true, our anxiety is quite understandable. Perhaps it comes down to damaged self-esteem and striving to boost it by seeking acceptance from others. Most people do this to some extent ... it is healthy to want to relate to and be accepted by others. Threaten that acceptance, and people can feel tremendously insecure. Still working on this whole thing myself.