Compulsive behavior is behavior which a person does "compulsively", i.e., not because they enjoy it but because they feel they "have to". The two most common forms are:
obsessive-compulsive disorder - obsessive, distressing, intrusive thoughts and related compulsions which attempt to neutralize the obsessions
drug addiction - a condition where a person takes a drug compulsively, despite potential harm to themselves, or their desire to stop
See also
psychological addiction => addiction - the repeated use of substances or behaviors despite clear evidence of morbidity secondary to such use.