I don't SI (ok at least not physically) but most of my friends do and I have a riend who was a therapist with a lot of people who SI. From what I have heard it releases endorphins and is addictive and some times our addictions can resurface for "no" reason.

This part of what you said really caught my attention "And so I don't understand, and I'm trying to figure out why I have such stronger urges to harm myself when I can't find any emotional pain that would explain the behavior or urge." Sometimes when we don't hurt that in and of itself can be a trigger. Many us find more comfort in the familiar than we do in the safe/healthy. For so many of us here on PC we are SO used to pain that we strongly feel the urge to get back to our familiar place of pain.