Answer:
Some believe that there are certain people that are genetically predisposed to "addictive personalities." Chemical addiction, literally speaking, is caused by habit forming substances, most commonly substances that produce feelings of euphoria or pleasure, which after taken for a period of time, cause the user to have an obsession or compulsion to continue using that substance. In most cases the user is also physically dependent on these substance to function "normally". The brain becomes conditioned to operating with the substances and if it is not supplied it may result in "withdrawal" or with alcohol the "DTs" or "delerium tremens"
Others believe that the cause of addiction is the inability to deal with life as it is presented, out of that arises the need to alter that reality. While still others believe that it is a spiritual malady.
Addiction is hard to break for many reasons-
Biologically- the body's reaction to the lack of what ever substance is being abused, is usually extremely uncomfortable and makes the addict very sick. This causes fear in the addict and creates a motivation to continue use.
Mentally- The substances create euphoria and pleasure at onset of use. The addict also becomes accustomed to operating in daily life with that substance, and it becomes hard to function without it.
Socially- especially in regards to street drugs, usually an addicts entire life becomes revolved around the using, and finding ways and means of getting more of the substance. The friends of the addict are most commonly also addicts which makes the continued use acceptable and encouraged even.