Addiction can be seen as a way of coping/escaping reality. Addiction presents itself in various forms such as drugs, alcohol, food, sex, tv, shopping etc... People who find themselves addicted to something do not do it out of choice. No one wakes up one day and decides that they are going to be addicted to heroin.
There is usually something within that individuals reality that has become too difficult to bare that they just pre-occupy their mind immediately and frequently, so not to feel. Addiction can start out as something very pleasurable but then just becomes away of avoiding painful feelings.
It is not the drug itself that has become addictive but the turning away from pain and life that has become so addictive. The drug or drop of alcohol is just the vessel used to avoid feeling.