To be fair, this is the very thing that makes mental illness ... mental illness. It isn't that normal folks don't have some abnormalities and illnesses, but the fact that it affects their lives and prevents, to some extent, them taking part in a normal life. Sometimes it plagues folks.
In a way, it is kind of like blood pressure. Everyone has blood pressure, some higher than others. A few people that run a little high don't need to do anything. Some people take meds, and some do diet and exercise and such things. This all depends on what their blood pressure responds to and what the cause is - some folks get it genetically, others through diet and lack of exercise.
Edit: It is an easy example only, not trying to minimize mental health issues.