People give all sorts of reasons for using drugs. Some just want to have fun. That's fine until it stops being funny. When that happens there is something else going on. It may be physical or psychological dependence.
Physical dependence is straightforward.
After a while without it things go back to normal. Psychological dependence is different. It is when a person believes he needs a drug. It means that life isn't quite right without it. That things don't work as they should.