I expect to write or compile some research here so this is the start.

http://www.psychologytoday.com/basics/dopamine

Dopamine is a neurotransmitter that helps control the brain’s reward and pleasure centers. Dopamine also helps regulate movement and emotional responses, and it enables us not only to see rewards, but to take action to move toward them…people with low dopamine activity may be more prone to addiction. The presence of a certain kind of dopamine receptor is also associated with sensation-seeking.