Most borderlines are quite intelligent in terms of their intellectual capacity.
Most borderlines lack the corresponding emotional intelligence.
As I've said before a big part of relationships is one's emotional maturity and stability. Here is where many borderlines hit that glass wall. From behind this glass wall one really can see and intellectually be aware of what others are experiencing and sharing with one another. The problem is that this knowledge or awareness is not always consistent. It also often cannot be translated through the wall of glass.