Deviance is considered a social construct because it is defined strictly by what a particular society defines as normal. What is deviant in one society is not necessarily deviant in another.
Various societies throughout the world define themselves by a set of shared common values. These values form the basis for normality within those societies, but they do not necessarily have any influence on perception outside of those societies. The things that do not fall within that normality become a deviance.
Those who are deviant are perceived as outside the system of common values and outside of the norm. However, it is possible for values to differ within a single society based on certain factors. Older people in a society might see some behavior as deviant that younger people in a society do not. Location also plays a role in determining deviant behavior. People of one region may find a behavior deviant that people of another region do not. Someone's position in society also allows them to behave in a way that might be considered deviant by someone of lower rank. Sometimes, deviance is assigned to people based on appearance or physical deformity rather than how they behave. These are known as stigmas.