Addiction can profoundly affect the brain by altering its reward system and the way it processes pleasure. When someone repeatedly uses a substance or engages in addictive behaviors, it leads to changes in the brain's chemistry, specifically in the levels of neurotransmitters like dopamine. This alteration disrupts the brain's normal reward circuits, making it increasingly difficult for the individual to find enjoyment in everyday activities and leading to a dependency on the substance or behavior to feel good or even normal.