Insecurity seems more like a definition than an emotion. In the case of our described friend, he seems to have moved outside himself as a locus of identity and to be seeking it in others. Once your self-definition belongs to someone else, you ARE insecure. Not only that, the ability to trust that other person is eroded. It's a terrible burden to place on anyone.
So we come back to the question of our friend's insecurity. He has basic needs that were not met. The more frantic are his efforts to get them met, the more pain he feels and he seems to be somatizing that quite intensively.