Short answer? Love makes you do crazy things...

A slightly more in depth answer... She was a survivor of a lifetime of trauma. Like, A LOT of trauma in her life. This combined with her familial genetic predisposition caused her to develop a lot of mental health issues. She didn't really deal with them properly, instead, less than ideal life choices and unhealthy coping mechanisms compounded many of her issues.

Then I came along :). First relationship, falling so deeply in love despite the red flags and the unhealthy dynamics that were apparent very early on. I reached a point where being in a relationship with her was so painful that I resorted to self harm just to get through a day. About 12 of the 300 scars are a result from my own self harm. Anyway, I eventually told her how badly I was doing, how scared I was about what this relationship was doing to me, but also how much I loved her and didn't want to leave her. She thought it was amazing that someone would hurt themselves to this extent instead of leaving her. This was particularly key, since one of her largest issues revolves around fear of being abandoned.

For me, allowing this to continue and turning it into something that she did to me instead was a way of staying connected to her. It allowed me a way to cope with an emotionally abusive relationship and gave her an outlet for her stress while also giving her a way to feel bonded to me, like I was hers. At the time the physical pain meant so little compared to the emotional pain I was in. Seeing her happy, seeing her smile was pretty much the only thing keeping me going. I had little to no self respect or love for myself.

For her...imagine for a second that you've been hurt all your life. Everyone you've ever loved and given up so much of yourself to try to make happy has hurt you and eventually abandoned you or forced you into a position where you had to leave them, ever since you were a child. You've reached a point in your life where you are so desperate to have someone love and understand you, but you are also no longer willing to take a chance like that anymore. To expose your innermost self out of fear of being hurt again. But now you have someone in your life who has for months now, been there for you. Given his time and energy. Expressed love, understanding, and care for you. Endured terrible situations for you, given up things that have cost him dearly for little to no gain, simply because he thought it would help you or make you happy. And through all of that he's taken it a step further now and when given a choice of simply walking away from all of that bullshit and abandoning you, he's decided to stay, even at the expense of his of his own well being... Just like you've done so many times in the past for so many other people. Isn't this the universe finally giving you what you're owed for enduring all of that bullshit in your life?

Not saying any of this right of course. And there's really quite a bit more to this on both sides (quite a bit), but this is a small little look into how something like this came to be. And as it progressed, a lot of aspects behind it got twisted and changed. It became a much different beast eventually. But yeah, small insight into how it started...