Further to this, the model doesn't believe that unhappiness is a "cause of" people drinking. There's a chapter called "Learned Connections" and this is a term the authors use to describe the relationship between what we think are the causes of our drinking, such as unhappiness, and our drinking itself.
It seems commonsensical to say that some people who are unhappy would drink if they think that would give them some level of happiness at least for a while. But that is not the same as saying that their unhappiness is "causing" them to drink - happy people drink too because they believe it will make them even happier.
So, the model says, being happy or unhappy are not causes or reasons to drink. Rather, the reason to drink is the same for both happy or unhappy people: to be happier than they currently are (the PDP)