People vary in how likely they are to be affected by drugs. For some people this could be because they are in a vulnerable position in society – maybe they don’t have a job, or they mix in circles where drug use is common. For other people they might have a biology that makes it hard for them to stop craving the fun bit of the drug, or hard for them to go through the withdrawel stage. It is also very likely to be a combination of both for many people.
Drugs can produce many different feelings and sensations in people: serenity, peace, indifference, numbness, fear, dizziness, an adrenaline rush etc. To some people these feelings can be quite pleasurable to experience and therefore might become addictive especially if they offer a sense of escapism. So the sensations some drugs produce become highly sought after by people and they then may go on to develop a dependency on a particular drug. I’m not an expert on drugs but I think that makes sense