How I view this issue is that we are all basically selfish in life, alcoholic or not. With that comes the good, the bad, and the ugly. Sometimes our wants coincide with another's wants, and at those times--good times happen, and pleasant memories are formed. When our wants differ from our partner's wants, conflict occurs, and this is where the bad memories form.
When we separate, we feel this loss and disconnect, and we start to experience withdrawal symptoms. Those symptoms often include remembering the best memories, and downplaying the worst memories. No doubt alcoholics do the same--when experiencing withdrawal--remember the best times drinking, and forget the worst.
There is an innate protective mechanism in our minds to not experience withdrawal, hence, loss.