Ans: In a One-To-Many relationship, one object is the “parent” and other is the “child”. The parent controls the presence of the child. In a Many-To-Many relationship, the existence of either type is dependent on something outside the both of them.
Subject matter (domain) should dictate whether or not the relationship is One-To-Many or Many-To-Many however making the relationship unidirectional or bidirectional is an engineering decision that trades off memory, processing, performance, etc.
Confusing is that a Many-To-Many Bidirectional relationship does not need to be symmetric. Naturally, it would, but symmetry is not a requirement.
Just remember that Hibernate or any other ORM doesn’t give a hoot about maintaining symmetry when managing bi-directional many-to-many relationships.