Mari is such a good girl, guess thats why shes such a big M, heard that people that have a lot of expectations on them or that always worry about other/are relied upon a lot, really like that feeling of just letting go completely and entrusting everything to another person, a sense of freedom of having another completely lead you while you dont have to worry about anything.
That other girl would also make a good M i guess too bad she never found another pervert to take charge for her like lucky Mari did.