Well there's different symptoms to have if your a drug addict one being the fact that he or she may be depressed. Also he/she may be sad because a lost love.
Becoming addcited to drugs or alcohol can often be a sign that a person has problems with the neural wiring in his or her brain. Some people seem to be more susceptible than others. In no case is addiciton which involves these substances useful for the person on a daily basis.
