A friend of mine has a son who is an addict. Has been in trouble since hi school with drink and mj. Now in his 30's and still has trouble with alcohol/addiction.
Both parents never did any kind of drugs and drank very little. I know both sets of grandparent and neither drank. And ggparents same. He has 3 other children and none of them drink.
Sometimes it is just the luck of the draw.