Responses
I'm pretty sure it's because what he was seeing. As you already mentioned we have a clear hint too (when he says he can't help but keep looking in the mirror) because all the other times jjk was hitting the prostate but still his dick remained flaccid.
i’m rereading bc the uncensored chapters came out. but i’m so curious abt chap 20. we all know that dan has trauma from being sexual assaulted by an ex-coworker, which leads him to Never having any type of experience with sex, knowledge abt sex, etc until he met joo dickhead. in chap 20, they’re having mirror sex and dan mentions that he can’t look away from seeing himself being impaled, and he pops a boner and cums which is different from usual bc when they usually have sex, he’s flaccid. so i was wondering if he was subconsciously turned on by what he was seeing, or if joo dickhead was hitting his prostate and it made him feel really good.