Dopamine is a neurotransmitter found in the brains of humans and many other animals. The first time that dopamine was synthesized was in 1910 by two scientists, George Barger and James Ewens. The name dopamine comes from the fact that it falls in the amine functional group. It's IUPAC name is 4-(2-aminoethyl)benzene-1,2-diol.
Dopamine plays a major role in the reward pathway of the brain. Essentially most rewarding behavior that has been studied has been found to increase levels of dopamine in the brain, especially in the nucleus accumbens. Increases in dopamine levels have especially been observed in activities such as eating, exercise, sexual activity, and listening to music. It has been hypothesized that the rewarding feelings that we get from dopamine have helped contribute to evolutionarily adaptive behaviors. It just so happens that many of the behaviors that are crucial to our survival also give us pleasure. Since we are rewarded by these feelings of pleasure, it is more likely that we will continue to take part in these life sustaining behaviors. Dopamine has also been linked with implicit learning which could be another contributing factor to it's effects on the reward pathway.
Diseases and conditions such as Parkinson's disease and Schizophrenia have been linked with irregularities in the dopamine system. Dopamine also has effects on the body outside of the nervous system including the digestive system and endocrine system. It is also sometimes used in order to stimulate the heart in cases were babies are born without a pulse. Certain anti-psychotic drugs have also been known to affect dopamine levels.
The reason I chose dopamine as my favorite molecule is because without dopamine giving us rewarding feelings, we might not have any favorites.