Dopamine is an important brain chemical involved in motor functions and general arousal. Dopamine is also involved in the ability to learn and the encoding of stimuli. Virtually all drugs of abuse, including heroin and other opiates, alcohol, cocaine, amphetamine and nicotine activate dopamine neuronal systems.
So called "natural" rewards such as food, positive social interactions and even humor, likewise activate dopamine neurons and are powerful aids to attention and learning. Sweet solutions are a well-characterized natural reward. When a source of sugar is encountered, animals will consume substantial amounts, return to it preferentially, and will work to obtain access.
A number of psychiatric disorders are associated with imbalances with dopamine, particularly schizophrenia, Parkinson’s disease, and mood disorders, including certain types of depression. Dopamine may also be involved in restless legs syndrome.
http://www.raysahelian.com/dopamine.html