The definition of a loner I believe is someone who prefers to be by themselves instead of with a group of people. I think there are many levels of being a loner and most loners actually interact with people on a daily basis whether they want to or not. They know that they have to interact with people in order to survive.
I don’t think there’s anything wrong with being a loner because you may prefer to do things in isolation but that doesn’t mean that you shouldn’t have contact with no one. You should still interact with people whether it’s at work or during a social event. However if someone is being a loner and they have a mental illness then a family member or friend should make sure to check on them occasionally.