I don't think Iori is a masochist, per say, because sure, she wants Kanan to hurt her, but I think her situation is more Borderline PD. Or, at least, that's what it strikes me as lol.
If I recall some of the main characteristics of Borderline are fear of abandonment and self-destructive behavior to get attention, and I'm pretty sure Iori has both of those.
Or maybe I totally missed something and it's because (like some other comments have said) of an abusive relationship in the past.
Either way, I like to feel smart with my knowledge from AP Psych so hopefully it's the former lololol