The brain's "reward circuit", which is responsible for making people feel good and flooding it with dopamine, is what most drugs affect. People who have a good reward system will eat well and spend time with their loved ones. People will continue to do things that are not fun, but that are good for them, such as using drugs. This encourages people to continue doing the same thing.
The brain adapts to drugs by making the reward circuit more ineffective. This will continue as long as the person continues to use drugs. Tolerance is a term that makes people feel less high than when they first used the drug. You could take in more of the drug to achieve the same effect. These brain changes can cause a person to lose interest in activities they once enjoyed, such as eating or sexual relationships.
Other chemical systems and brain circuits can also be affected by long-term drug use. This can have a negative impact on learning, judgement, decision making, memory, behaviour, stress, judgement, and decision-making. Many people who abuse drugs continue to use them despite knowing the potential dangers.
Why is it that some people become addicted to drugs and others don't? There are many signs that someone will become dependent on drugs. There are many factors that influence the likelihood of someone becoming addicted to drugs. A person's risk factors will influence how likely they are to use drugs to get addicted.