A sick psyche is a closed system, like a lake infested with algae because the circulation of water is cut off. A healthy psyche is permeable, mutable, and liable to change and exchange, feeding on what supports life. Like all such systems, psychological health depends on the capacity to endure, which is the definition of sustainability. That capacity is enhanced whenever there is pleasure.
A horse won’t drink polluted water and will walk long distances to find another water hole; but a closed, sick psyche has lost that instinct, which explains why giving good advice inevitably fails with individuals who have come more or less adapted to toxic relationships. Their unsustainable relationships are the invested lake they call home.
An algae does not have the capacity to move out of the lake while a human does but before that person can move away from toxicity, the instinct that prevents the horse from drinking polluted water has to be reactivated. This is one way of describing the goal of psychotherapy; I like to imagine our lifelong quest for wisdom as a continuous process of detoxification, a moving away from situations and relationships that can’t support life.