The ultimate goal of all medicine is to treat the cause and to cure or completely heal the patient. This question must be coming from the point of view that takes into account a practice of medicine regarding mental health where the doctor is following a process of identifying symptoms first and beginning treatment before a definite cause has been found.
As has been pointed out, this is not always possible, but by treating symptoms in certain ways, doctors work toward a better understanding of the cause (if the treatment eliminates some but not all of the symptoms).
I certainly agree with this statement, not only in cases of addiction, but in all cases of mental and physical illness. Unfortunately, we are not able to find the cause of many mental and physical illnesses, and thus, all we can do is address the symptoms. In the case of mental illness in particular, there is insufficient funding to do all the research that should be done, to some degree because mental illness is so stigmatized and because many people believe that if only people would "straighten out," they would get better. This is particularly true with the illness of addiction, a kind of "blame the victim" mentality that seems to prevail, at least in the United States. There is no doubt that an addict must take some responsibility for his or her own recovery, but there is also no doubt that this is a real disease.
As litteacher points out, this is not always possible. In the best case scenario, the best way to treat any condition would be to find the underlying cause. That said, some people may simply have an addiction without having any underlying condition. In these cases, one would spend far more time trying to find a cause, which does not exist, and not enough time treating the problem.
Sometimes it is not possible to find the underlying cause of the addiction, and sometimes there really isn't one. Even if you can find one cause, you may not be able to treat it. For instance, if a person is an addict because of family history, there is not much you can do.