That’s an incredibly complicated question, and I’m in the mood for simple answers. So, bearing that in mind:
Healthy possessiveness is a groove; it’s a delicate, deliberate path someone etches into the world for you to travel. It’s a direction to which you apply momentum.
Unhealthy possessiveness is a rut; it’s a cold, sloppy mess that sucks you in, freezes you in place, and often requires help to escape. It doesn’t want you to move at all.
Some girls realize this and behave accordingly.
Some girls go muddin’.