If someone is a loner, does it mean they have a mental disorder

Others respond very poorly to criticism or perceived criticism from people, and so they choose to be loners than to deal with possible criticism. These people are considered to have avoidant personality disorder.

Other person choose to have only one or two friends, and they often derive a lot of their identity and self-worth from these individuals. Oftentimes, they are very close with their mothers and fathers, even as adults. Some would consider them to have dependent personality disorder.

There are people who believe that others are "out to get them," and oftentimes there is a grain of truth in what they say. For example, dealing with a slick salesman leads them to think that everyone in the world can't be trusted, when most people would think that it was the salesman. Yet, it isn't an outlandish thought, like aliens are impersonating people. These people are thought to have paranoid personality disorder. The latter--the ones with outlandish thoughts and perceptions--generally fit into the schizophrenia group of mental illness, which are best treated with medication as well as social support.

With the personality disorders, it is important to get help for these people, because they often don't see anything wrong. It is also very difficult to treat them, because again, they don't see anything wrong.

In summary, the reason they are considered disorders by mental health professionals is that they are maladaptive, and they interfere with their lives in a significant way. Unfortunately, treating personality disorders is extremely difficult.