The brain releases a neurotransmitter called dopamine when a person experiences rewarding situations, particularly in behavioral addictions like eating disorders, sex addiction, addictive gambling and addictive video game playing. Dopamine is also associated with drug and alcohol abuse.
As the rewarding experiences continue, the neuronal connections and memory connections associated with the euphoric experiences are strengthened and remodel the connections in the brain. When a video game addict stops gaming, the brain loses the stimuli that releases the same rewarding highs associated with the dopamine rush. The brain also adapts to the high levels of dopamine by developing what is known as tolerance. The brain produces less dopamine and/or reduces the number of dopamine receptors in the reward circuit, which decreases the enjoyment of the drugs and addictive behavior, requiring the addict to increase the drug or behavior to feel good and to feel normal. Otherwise, the video game addict can suffer from depression and physical withdrawals when they stop playing games. Data presented at the British Psychological Society conference in Dublin showed that video game addicts missed meals and went without sleep to spend more time playing, and they suffered withdrawal symptoms and had difficulties cutting back; additionally, they were found to be more introverted, emotionally unstable and had lower self-esteem than average.
Presumably addicts' brains have built up the ability to produce and release dopamine associated with addictive behaviors. There is growing research evidence that dopamine is released with video game play.,  The link between dopamine and video game play was published in the well-respected scientific journal, Nature, and the Journal of Addiction Medicine.
The following video from pleasureunwoven.com explains the role of dopamine in addictions.
 National Institute of Drug Abuse. “NIDA InfoFacts: Understanding Drug Abuse and Addiction.” http://www.nida.nih.gov/infofacts/understand.html Accessed September 4, 2011.
 Fleming, Nic. “Video game addiction 'like being on drugs’.” The Telegraph. April 3, 2008. http://www.telegraph.co.uk/earth/earthnews/3338422/Video-game-addiction-like-being-on-drugs.html. Accessed September 2, 2011.
 Koepp et al. “Evidence for striatal dopamine release during a video game.” Nature. 1998 May 21;393(6682):266-8.
 Han et al. “Dopamine genes and reward dependence in adolescents with excessive internet video game play.” J Addict Med. 2007 Sep;1(3):133-8.