The brain can experience pleasure from all sorts of things we like to do in life; eat a piece of cake, have a sexual encounter, play a video game. The way the brain signals pleasure is through the release of a neurotransmitter (a chemical messenger) called dopamine into the nucleus accumbens, the brains pleasure center.
This is generally a good thing; it ensures that people will seek out things needed for survival. But, drugs of misuse such as nicotine, alcohol, and heroin also cause the release of dopamine in the nucleus accumbens, and in some cases, these drugs cause much more dopamine release than natural, non-drug rewards.
Substances Trick The Brain's Reward System
Addictive drugs can provide a shortcut to the brains reward system by flooding the nucleus accumbens with dopamine. And, addictive drugs can release two to 10 times the amount of dopamine that natural rewards do, and they do it more quickly and more reliably (helpguide.org).
When Tolerance Develops People Need More
Over time, drugs become less rewarding, and craving for the drug takes over. The brain adapts to the effects of the drug (an effect known as tolerance), and because of these brain adaptations, dopamine has less impact. People who develop an addiction find that the drug no longer gives them as much pleasure as it used to, and that they have to take greater amounts of the drug more frequently to feel high.
Below is a picture (helpguide.org) of the brain and the nucleus accumbens, in addition to some other brain regions that are affected by addition.
And, here is a picture from Recovery Research Institute's own research of the brain activated by alcohol (Gilman et al., 2008). The nucleus accumbens is activated.
There is a distinction between liking and wanting the drug; as over time, the liking decreases and the wanting increases. Individuals with a substance use disorder continue to seek and use the substance despite the negative consequences and tremendous problems caused for themselves and for their loved ones, because the substance allows them to simply feel normal.