They are weak. That's all it is - weakness. And because they are weak, they can't cope with the stresses and strains that come with life with a child with issues (or even that come with just the normal stresses of life, for many of our spouses). They become depressed, and then they become addicted to the "high" they find in a new relationship. Then, just like a cocaine addict, their behavior becomes all about seeking the "high" and they don't see the destruction their behavior is causing for their loved ones.
That weakness may be a moral failing, or a biological weakness, or both. In retrospect, my ex had all the signs of a dopamine addict. It's just when his method of obtaining that dopamine "high" switched from adventure sports to affairs that our marriage suffered.
The good news is, those of us who come here, are strong. Stronger than you know. We are the ones who stick it out through tough times. We are the ones who are our children's rocks.
