Why did i have an affair? i am male.?

i have not figured out why i had an affair and consequently destroyed my relationship with the one woman i have always loved and always will love. i had a perfect life, why the hell did i do it.i cant find the answer and its torturing me.i need to give my ex partner a honest answer to settle her mind and i dont know why i did it.any ideas as to why i should have done such a stupid thing?