Mental illness is a sign that who we are does not fit with our circumstances
The circumstances/environment you are in have a big impact on your mental health. This coincides with Behaviorism and Social Learning Theory, where the main idea is, that our behavior is shaped either by our environment or by observed behavior from others.
The positive counterpart to this might be James Clear’s statement that Cues for certain actions are often related to an environment, and environments can be designed a negative environment—be it in your living or the social environment—can invoke negative actions and lead to mental illness. This would mean, that we could design our environment in a way that helps us to solve our (mental) issues or at least lessen them somewhat.
The “who we are” part connects to the idea that we can somewhat choose our own identity. If this is true we could either work on the environment (to the degree we can) or we could reframe and/or change our own identity.