This is a tough one.
My mother left me and my sister when we were 13 and 15, she was having an affair for 8 years with a strange man who lived around the corner, and eventually told my dad, who forgave her but said she could not see this other man again.
Well she couldn't keep to that, so one day she just left. my father had never been very hands on, and was now left with 2 teeanage girls and working full tiem, so it was hard on him. but unfortunatley he did not step up to the plate - so to speak.
my mother kept in contact with us, but did not have custody, we saw her once a week for a couple of hours.
my father divorced my mother and remarried within 6 months to a woman not much older than my sister (she was in her 20's) and my sister and myself were put to live in a caravan at the bottom of the garden while he and his new wife lived in the house with her son.
I cannot believe it as I am writing this how we could have been treated in this way.
basically my mother left because of unhappiness, my father was a very cold and unfeeling type, and still is, so she didn't get any affection from him, she was a wonderful mother up until that point, and I never suspected she was having an affair, she was always there for us.
she left this man after about 6 months, and turned her life around by becoming a born again christian, we moved back in with her after 2 years of neglect and ignorance from my father and stepmother, I totally forgave her, she totally made up for what she did, she died 4 years ago, and I miss her chronically, she was only 58.
there you have it - one reason - unhappiness and emotional neglect will make you do things like that.