I guess it’s different for everyone, for some it may be a one off traumatic incident and for others the drip drip of life.
I’m the latter, it was the drip drip of an unloved childhood with a mother who was suicidal and a father who chased other woman and put his children below his latest bit of fluff.
At the time I guess it seemed normal. Didn’t all children grow up this way? It was only years later when my mental health got progressively worse until I was suicidal did I realise after counselling that the seed was sowed in my childhood and had grown over the years to the darkness that is now part of my life.
The lesson to me is that the formative years are so important to children. Show them love and show that you care. We all need that.