There are certain people who seem to equate the two. They have have nothing to do with each other! Granted, if you have a mental disease, that can very easily lead to addiction but it is not the cause of it. The cause of it is drowning yourself in booze or sticking needles in your arm on a daily basis.
Mental disease is NOT a choice whereas addiction, at the end of the day, is. No you don't choose to become addicted but to continue to be addicted, IS a choice.