If you have never been able to count on people being there for you when you need them, then it would make sense to be afraid that they won't be there for you now either. It's somewhat typical human behavior that sometimes it feels like we are too distant and alone, so we move closer together, and then we feel suffocated and overwhelmed, so we push away. For borderlines, it's like that but more extreme. I think that a lot of it has to do with past experience and how we learned to cope with it. The key to overcoming it is to learn to recognize when something in the present feels like something from the past, and we are likely to react as if it were the very same thing, when actually there may be some big differences and how we reacted back then isn't the best response now. So we need to learn to be in the present and act according to what is in our best interests now. Easier said than done, though, isn't it? It takes a lot of self-awareness and practice.