I'm diagnosed with BPD and I've had, i think, some experience with psychosis and dissociation, although I think I'm probably on the mild end of the spectrum mentioned.
Not being an expert, but i'll venture an opionion: I think (for me at least) it's related to like freezing with fright, in a survival situation an prey animal's mind can just shut down in intense panic. So it seems to me like my experience of dissociation has been emotion driven.
What I've experienced most commonly is feelings of not being there, not being present, or being invisible, as if nobody else can see me. I've found that reconnecting with my senses by smelling things (flowers, tomatoes, perfume) can help!
I've also experienced feelings of everything being unreal, or of impending doom, and free floating anxiety and fear that doesn't have any obvious attachment (but often siezes on one). I think these feelings and ideas can be related to dissociation, but dissociation (for me) can be like the psychosis/anxiety (or unreality) attaches to my sense of self rather than to external things.
I can defiinitely understand why people might seek out pain as a way of confirming their reality and sense of self! And if the feelings of impending doom and trapped despair go on long enough you can get to wishing it would just like, get on with it...
Those are my feelings, anyway!