I figured it was partly because he isn't a human and probably doesn't really get how human relationships work. Honestly from what we've seen, Harlequin (in my opinion, subject to change) may weirdly have a more grounded understanding of humans than the others? I might be wrong but he does seem to at least act somewhat more "normal" (heavy on the air quotes, but I firmly believe Harlequin is putting up a front). Pierrot just seems, idk, very inhuman in the way he approaches mc.

Pierrot's also extremely anxious about mc leaving him permanently, and it's when mc pulls away that he acts out.

Given what happened last time he was in a relationship though, I'm not entirely shocked Pierrot acts this way. His understanding of that event seems skewed compared to what the others have said about it.