1) maybe it is really true. They are not ready for a relationship, or the person is having a negative reaction to something neutral about the other. And that person, who is ending, is insighted enough to realize that it is not the other persons fault, it is their own stuff. <br />
2) It is also a handy phrase when dealing with someone with a fragile ego because you don't want to make that person more unstable.
It means that the person saying this doesn't really mean that it's themselves most of the time. Also it could really be true/