It is not possible to define one reason people commit crimes, as there are many variables involved in determining why people commit criminal behaviors. Most certainly, the reasons people commit petty crimes (such as shoplifting, dealing in drug use and abuse, and vandalism) vary from the reasons people commit horrendous crimes (such as mass murder).
Classical theory of crime
People commit crimes because they choose to commit crimes. People take the risk to commit crime, even though they are fully aware of the consequences. Criminals often know that the choices they are making are wrong or immoral, but they choose to commit the crime knowing there may be repercussions, but thinking they will never get caught.
Interactionist theory of crime
People commit crimes because the people they associate with (or interact with) commit crimes. When people are a part of a group, they want to be accepted. If the group is committing criminal acts, people associating with that group are more likely to choose criminal behavior as well. Criminals in this category often have poor role models and lack self-control, leading them to end up with the “wrong crowd.”
Sociological theory of crime
People commit crimes because they live in (or are surrounded by) an environment that encourages criminal behavior. For instance, a young male growing up in the inner city may have been surrounded by crime his entire life. He may see more-mature men making money by selling drugs, or he may see one of his friends coming home with a new car that he stole. In this instance, his surroundings do not allow him to see the virtue of living a criminal-free life. He sees rewards and benefits from criminal behavior, and he sees no reason that he should work long hours at a factory job when he can go sell a few drugs down the street and make the same amount of money. His criminal environment may be all he knows, and the likelihood he will commit crime is high.
Biological theory of crime
People who commit violent crimes are sometimes predisposed to crime because of their biological makeup. These are people who are mentally incompetent and may not have known the reasons for committing the heinous crimes they commit. They are biologically disposed to committing crime. Some people may be born with this predisposition and are genetically wired that way. Others may be impacted by changes in hormone levels, physical trauma to the brain or a fluctuating body chemistry. In some cases of violent crime, criminals have stated they felt even in childhood they had compulsions to commit criminal acts. They may have picked on other children or tortured animals. When they became adults, they felt they could no longer control those compulsions and acted upon them.