Two things. What she says was the most motivating one is oddly prosaic.
She is a veterinarian. She has loved animals her entire life, considers them her guardian angels. We had finally moved into a rental house that allowed pets, so we got a dog, an Australian Shepherd rescue dog who turned out to be the best dog who ever lived. I took her all the way through advanced obedience classes. I don't really have the words to express what a wonderful animal she was; in my mind, she was the Platonic ideal of a mother sheepdog. She also had distinct ideas of propriety, a phrase that could only make sense if you have ever had a female Aussie, and as a consequence was about as low maintenance as a high energy dog could be.
One day my wife looked at the dog lying next to her on the bed and thought “You're nothing but trouble, I wish you were dead”. The next second she realized that that thought was so unlike her, and so different from the reality of our dog, that it was clear something was wrong in her head.
The other was that at the same time we had had a daughter, a beautiful baby girl. My wife started taking her anger out on the baby as well as me. To be clear, this was not physical - but she resented our daughter crying and so on, and was brusque and short tempered with her. She was introspective enough to recognize that this was a problem.
So some combination of those two experiences acted as sort of an intervention.