One of my professors in college described people with personality disorders pretty well. He said they want to FEEL better and not to actually GET better. GETTING better takes a crapload of pain and hard work and soul searching and most of the time people with personality disorders are too weak willed and inconsistent to be able to effect that kind of change in their lives.
It's easier for them to just pop some pills and keep doing the "I love you, I love you, goodbye," thing that always seems to happen. Borderlines especially are impulsive beyond belief and you always have the headdesk or facepalm moments around them. It never ceased to amaze me the amount of stupid crap my borderline would get himself into. Then, he would have people around him have to pick up after him. Then a few weeks later, after they bust their bottoms to make sure he is safe and healthy, he throws them out of his life and/or threatens them, telling them they're a terrible person and they've always been a bad influence on him.
And what do you know, lately he has tried to use a mutual friend to be friends with us again. I've finally learned my lesson with him though. I'm being very firm and I will not have him in my life again, friend or otherwise. He's too dangerous and he causes way too much drama and turmoil.
Unfortunately, when BPDs are "up" they can be amazing people to be around. They're intoxicating and captivating. You never see the storm coming and once it happens it is too late. Unfortunately, since most reasonable people expect others to be mostly reasonable people, it takes a long time to figure out that a borderline is never going to change. They will appear to change. They will be very sincere in their apologies. Mine even let me come out and confront him with the fact that I thought he had borderline and he was very receptive and apologetic and said he was going to work on changing and that he didn't want to hurt people anymore.
But then they get anxious or angry, and all that talk of change just goes right out the window and they are just as sincere when they tell you they hate you and that you have no idea what you're talking about and this is just how they FEEL and it is never going to change.
Amusingly enough, he frequently uses the logical/emotional gender stereotypes on himself and his girlfriends, but he is one of the least logical and most emotional people I know.