While genetic factors have a role to play, environmental factors are also important; indeed it is often difficulty to disentangle the causal factors in the development of addiction.
If you would like more information about the development of addiction you may like to read the following short insight into the possible genetic and environmental factors involved in the development of Alcoholism. Free kindle edition available till 24th of September, 2012.
http://www.amazon.com/Alcoholism-Environmental-Writings-Psychology-ebook/dp/B009D01N86