For most people, the initial decision to take drugs is voluntary. But as they are swept up into the cycle of addiction, the neural pathways in their brain change so they are less able to control their behaviour and resist their intense impulses.
It works like this: the brain rewards pleasurable experiences (such as food, intimacy, and laughter) with surges of feel-good chemicals like dopamine. But using drugs triggers the release of much more dopamine than chocolate or cuddling does, and the rush of euphoria compels them to repeat the experience. The more someone uses drugs, the more they condition their brain to anticipate the same substance-fuelled pleasant sensations.
That’s why it’s so difficult to stop. The brain becomes wired for addiction. Eventually, one’s tolerance may build so much that addictive behavior no longer provides any pleasure, and using drugs simply becomes a way to avoid withdrawal.