Probably to keep it like some kinda treasure.
Well probably because he is very possessive of choi moonhyuk and probably dislikes the fact that choi moonhyuk cut off his pinky in hopes that his ex-lover at that time would be alive. And seonjin being the princess that he is dislikes that choi moonhyuk gave up a part of his body for someone else and hence he wants the pinky back because seonjin wants to possess all of choi moonhyuk.ლ(´ڡ`ლ)
You explained that better than me lmao
woww make sense
Why u want the pinky bruh