To me, the difference seems to be between "content" and "discontent". I am generally content with whatever I'm doing in life, whether it's skydiving or sitting on the couch watching baseball. To the BPD, there is never any content moment - really sad to think about, and must be a hard way to go through life.
Yes, this is fascinating! I wonder if that's why all they want to do is plan stuff for the future... . my BPD ex was never content just doing "nothing". If I came in from work, she had to know what the plan was for that evening. She could never "Just be". Even if it was watching a DVD together, it had to be planned. Or even if it was just "chill time", playing on facebook or whatever, it had to be stated that that was what we'd be doing. She couldn't handle just letting the evening unfold.
I completely agree that they can't just have contentment in the moment.
Looking back now, I'm not even sure we had that much in common. We felt like we did, as we talked continuously. About our upcoming wedding, about days out we'd be doing, about our future children. But living in the moment... . we just were so very different.
It's fascinating, all these similarities between stories.