What are reasons people start drugs?

Perhaps your question would be best Worded,

Why do people start abusing substances?

People are on habituating drugs for legit reasons. There are several classes of habituative drugs. There are also drugs which are habituative that do not fall in to the class of drugs of abuse. Anticonvulants, for example. Most Anticonvulsant drugs are habituative. They do not fall in to categories opiates, barbiturates, amphetamines. Sudden withdraw from these drugs is fatal. Drugs such as predisone can not be suddenly withdrawn either.

People abuse drugs (and substances) for many reasons. Medical marijuana can be one of them. Marijuana is a useful drug in treating epilepsy. But a high level, THC triggers seizures. Therefor a low THC strain must be used as well as a safe dose. Marujnana causes euporia. The patient will seek the eurphoria and take a larger (unsafe dose) and will seek high TCH strains of the plant.

People also use substances which are not drugs.