In my opinion, addiction is in more cases than not, a RESPONSE to some form of underlying pain or suffering. It is our human nature to avoid or reduce things that are hurting us, the fight or flight response if you will. Well adjusted people use drugs and alcohol because it makes them feel happier. In the case of people who are suffering from severe cases of stress, anxiety, PTSD or depression, using to become happier really is no longer the point. In order to become happier, you need to have some baseline of happiness to begin with. No, people who use for severe cases of the aforementioned conditions are using simply to be less MISERABLE. One could argue that seeking to be less miserable
is the same as seeking to be happier but I'd say that's a stretch.

Now one could argue that high levels of stress or anxiety, PTSD and depression are diseases or at the very least debilitating conditions and it is understandable that one would do whatever is necessary to mitigate or reduce the effects of those conditions or to become less miserable if you will. How one CHOOSES to try and remedy those effects vary from such things as therapy, group support, psych meds, education, or doing nothing at all and trying to ride it out.

So whether you use alcohol/drugs to feel better or happier or merely to feel less miserable and cope, it is still, at the end of the day, a choice that one makes in an attempt to change the way that they feel. It is not a disease, it is a choice and it's not hard to understand why people seeking to alter their present state, would choose that RESPONSE.