I am disappointed by this thread.
Also, I don't like you.
I think it's related to the fear of Death. Most people feel that they will cease to exist if other people don't appreciate/value/care for them. They are wrong.
It might be because, as Jennifer has suggested, they haven't developed their own ego/boundaries appropriately. So, they can't be certain where their needs end and the needs/requirements of others begin. The whole thing gets blurry.
I think people tried to bully me at school, I didn't even notice at the time. It was a frustrating experience for them. People have tried to bully me at work. I usually manage to get them fired or expose them in other ways. Revenge is sweet.
My advice: Stop fearing Death!
(and change your X to an I)