As Barbara notes here, one component of it is a fear of losing one's self - but there is another component. And it's a type of hubris, greed, and narcissism, based on a denial of a fundamental self-evident observation of human behavior in the human condition, as succinctly put by R. D. Laing:
I see you, and you see me. I experience you, and you experience me. I see your behaviour. You see my behaviour. But I do not and never have and never will see your experience of me. Just as you cannot "see" my experience of you. My experience of you is not "inside" me. It is simply you, as I experience you. And I do not experience you as inside me. Similarly, I take it that you do not experience me as inside you.
"My experience of you" is just another form of words for "you-as-l-experience-you", and "your experience of me" equals "me-as-you-experience-me". Your experience of me is not inside you and my experience of you is not inside me, but your experience of me is invisible to me and my experience of you is invisible to you.
I cannot experience your experience. You cannot experience my experience. We are both invisible men. All men are invisible to one another. Experience used to be called The Soul. Experience as invisibility of man to man is at the same time more evident than anything. Only experience is evident. Experience is the only evidence.
We can take "experience" for Buddhist purposes to mean one's own collection of the 5 aggregates and the various forms of consciousness. When I declare that your experience invalid because of either my experience or some external to both of our experiences (such as somebody's opinion of "scripture" or what someone was told "God's intention" or "God's words" were, this is a statement against your very being. In the case where either of us are citing something external to invalidate both of our experiences, this is a statement directed against both of us.
Although we can "see areas light up in the brain" corresponding to all kinds of human thoughts, feelings, emotions, hallucinations, volitions and sensations, these can never be the equivalent if any person actually experiencing those thoughts, feelings, sensations, emotions, hallucinaitons, and volitions.
Despite what I have experienced in my life, I really don't have a clue as to why Lindsay Lohan is messed up, or what makes a fundamentalist tick, what's in Eddie Long's brain, or any of a thousand other such questions. Only the principals know what's in their hearts and minds. True, there is empathy and compassion, but this empathy and compassion is counterfeit if it does not take into account that what another is experiencing is really experienced..
It is this invalidation of others that is the beginning of all kinds of religious exploitation, and all the big religions have done it from time to time. I practice Buddhism in part because by placing the issues with regard to the Way on the individual, at least in my school, much of this harm can be avoided. And that's another reason, as I say, that such practices are the last best hope for religion. Maybe Taoism and Jainism are too.