Some people are just masochists. My first serious girlfriend was as happy and healthy as any teenager, but was very similar to this. She bought a nice collar and we picked out a leash and we'd go out just like the couple in the article -- and it was all her idea. She got off on pain, I don't recall her ever having an orgasm where I wasn't choking her (she'd even cum just from getting throatfucked), and she obeyed. And then she cut off the relationship because I was too smart for her, and it made her feel inadequate (her words, not mine).

While I can see how this kind of lifestyle could possibly stem from self esteem issues, it could also just be what she enjoys.