This topic also interests me!
A few years ago, The New York Times wrote an article on single women with dogs. It quoted lots of psychologists, etc. The thought was more women are delaying having kids (or even husband) with careers, etc and that the dog was their defacto kid. They can put their motherly instincts on the dog, with less commitment than a family.
I think some girls have a dog as a regular pet, which is healthy. But most girls personify their dogs. I see this more than 90% of the time. Usually, women with serious professions without a husband, kids, etc. When a woman has a little dog, it's usually more red flag than a big dog. When a woman personifies her dog with clothing, birthday parties, play dates, gourmet treats, an Instagram acct..,,that is a major red flag. The chick can't have a successful relationship with people so the little dog is a surrogate.
I think cats is an even bigger red flag and has been for generations. Psychologists say BPD/HPD loves cats because the cat needs them for food, etc but doesn't smother them. The cat will love on them briefly but then leave triggering abondment!, but will ALWAYS come back. It has to for food. The cat will then leave before smothering them. Beware the cat lady. A buddy of mine got burned so many times, he would literally stand up at dinner and walk out if a girl mentioned her cat, total ghost.
Pets give unconditional love, which people with mental issues crave.