Why do people get addicted to drugs?Watch
As to why there are many many many reasons from genetic predisposition, simple time of taking it (although theres a big difference between addicted and dependent), stress and trauma make people more susceptible etc. etc.
Never mind the drug itself is a big factor, with weed not really being addicted vs. heroin being pretty addictive - the method of ingestion also effects it with injecting/smoking making addiction much more likely vs. eating/drinking and so on.
There are many many excellent books on this topic (professor Nutt wrote some good ones) if you want to educate yourself go to them. Whatever you do do not read anything provided in the news or, indeed, by tghe government who are decidedly unhelpful if not outright liars on the matter - merely look at their drug classification system which has nothing to do with the science or health costs.