The reason they seem to leave so easily comes down to a simple loss of respect and a certain degree of contempt they have developed for you. There doesn't have to be a diagnosis of any disorder. If they come into the relationship extremely needy, and you refuse to stick to your guns by letting things develop on a more rational basis, you will eventually begin shoveling coal into her/his seemingly endless furnace. At first it makes them happy, or SO THEY THINK. True dialogue between me and my adorable ex:
Christmas of 2015 -
"Thank you so much for making me a part of you family's Christmas! I have nobody and your daughter has the BIGGEST HEART!

I will cherish the book she gave me forever! I love you so much T+++! Please rush straight over to me after work. I miss you already!"
Christmas of 2016 -
"Ok I'll say it as*&^le. First of all 6 hours with anybody's family is a LONG TIME! I have been spending the last 12 years doing holidays with my friends who have no families from MY BAR SCENE and you took me away from that! And just so you know... .your daughter is a selfish as%^&e JUST LIKE YOU! LOSE MY NUMBER DICKHEAD!"
That was a tough one to type out because I went back on my knees for more. Read "on my knees." I don't even kneel in church. Stroking her hand balling my eyes out her looking DOWN ON ME! Only one other person knows about the above exchange.
How did we get there? I'll tell you. In the beginning (three weeks) she said I was the hardest man to figure out that she's ever met. She said most of her boyfriends wanted to be with her every minute. She asked me if I was playing games and I sat down and told her I'm not playing games. I'm just in no rush. I just want my next relationship to be my last. She said thanks, now she knows how to conduct herself when she's out (mild threat).
By now, I was sensing something was very "off" about her. Things like BPD weren't on my radar. But for starters, on her bedroom walls were glamour photos of... .herself. Huge prints. In her living room were mostly pictures of... .herself. Some with her best friend others with her cats but they were pics of her. When we went out she had a habit of invading personal boundaries and attracting a lot of attention. Dead Winter in Buffalo, NY and it wouldn't be long before a shawl would come off bearing heavily tattooed shoulders, back, and chest. Every time we went out. Eyes would turn, and it wasn't long before people (mostly men) would come up and use the tattoos as a conversation point. Daily, slightly provocative selfies on Instagram from her office saying good morning with hearts, making tongues wag.
Her response to male attention seeking was funny; "I don't know why this happens to me." oh my god why wasn't I running?
It was becoming instilled in my head that this was the hottest woman I could ever find and one of the most desired women in the city. My boundaries were becoming things I was prepared to tweak.
A month into it I was seeing her quite a bit but not as often as she wanted. She pulled back and tried to end it based on a perceived lack of interest. I chased her down and got her back. You know where it goes from there. All of the stories are the same to a large degree. She became attached and needy and insanely jealous. Asked that we remove friends who are a threat to our relationship. Weird about that... .I felt good that she wanted to do that, but looking back, there honestly were no friends in MY life who were a threat. I know right from wrong. Who was lurking in her stall? Who knows... .no matter.
Hypocritical jealousy started. Went through every female Twitter follower of mine. She would say go be with xxxx, she's hotter than me I'll never be enough for you. My response? "You are the most beautiful girl in the world! How do you not know I feel that way? Christ! Stop breaking up with me over things that don't exist!"
"Please stop breaking up with me. All I want is a relationship I can count on! I'm begging you! I will never leave you and I think you know that!"
Call her BPD or call her typical... .those very real things I said are not very attractive. I essentially lost here there. I was BECOMING HER! We would fight; she would call her ex-fiancee for advice, knowing it bothered me. I became ridiculously jealous and she threw it in my face, along with the tiniest little incidents that were residing in her head that she never brought out. Now I'm "controlling" and "toxic" and "need help." You bet I did! I was all of those things.
Over the course of months I fed her endless well of neediness to the point that I lost myself. There were times when she would dump me and I would remain confident and she would say, "well it sounds like you're fine with this so I guess I'll just move on." Then I would beg... .
The only time she ever responded was when I raged back and insulted her. That was usually how I got her attention. In the beginning she said she doesn't fight in relationships. Ha ha. She was a f%^&ing professional insulter. Her exes... .alcoholics, attempted suicides, overweight, stopped working when she left... .
In the end she didn't have a shred of respect left for me and I left her with a letter telling her she was my hero. And I would still try again.
I throw this post out as a cautious reminder. If you want to keep someone like this in your life and you're running out of options, you may want to play the game their way and prepare yourself for a long bumpy road of torment.
I ask myself every day what kind of man would pursue a woman who insults his own daughter, his own flesh and blood. A cowardly one for sure.
I'll get responses of "don't be so hard on yourself" I'm sure. But i may need to dip into some masochistic self whipping.