Interesting question! To me, rejection seems like a value judgement about who I am, that I am unworthy and unacceptable. Rejection makes me feel sad and ashamed. Abandonment is scarier, a profound sort of disconnection from one person or even the human race. For me, abandonment can bring feelings of despair but rejection does not. I do not fear aloneness and quite enjoy being alone, but feeling abandoned is an awful feeling. Of course, rejection is no fun either!