Marijuana users are more likely to drink alcohol. It is absurd to assert that marijuana use leads to alcohol use, or vice-versa. The gateway theory has been repeatedly used as a pretext for marijuana prohibition. It has been discredited by studies which describe it as faulty logic with no basis in fact. There is a statistical association with alcohol use and violence.
Alcohol works on parts of the brain which affect self-control.
This is a biochemical explanation. Tobacco use over many years is statistically associated with cancer. This also is a biochemical association. Cocaine, Heroin, PCP and Methamphetamine use is statistically and causally related to a variety of medical complications including death. IV injection of drugs with contaminated syringes has a clear statistical association with increases in the transmission of the AIDS virus.