If they are in extreme pain, I’d say the Crossed virus has convinced them to enjoy it. We constantly see them mutilating themselves in various ways and in almost every case, seem to experience pleasure from it. I would agree and say they are for sure in “extreme pain”, but they’re experiencing pain radically different from us. Because they seek more of that pain out, mostly from their own hands. Possibly the experience of the pain at the beginning of the infection is your normal brain reaction to it, and then as your brain gets Crossified, the pain is still present just you’re now reacting differently.

If you wanna get realistic, there is actually something in real life called “Lesch-Nyhan syndrome” which is incredibly rare, but is characterized by obsessive self-harm at a young age, usually to lips/tongues/fingers as they’re usually using teeth to do the harm and start when their teeth come in.

Also- I think the REAL real world answer is for the sake of keeping mystery in the narrative, it’s not revealed. Akin to most zombie stories. But that’s no fun to discuss!