What you use grows stronger. This is as true for the brain as for physical muscles, and is called neuroplasticity. With repeated use, a thought, mood, or emotional state becomes a ‘habit’. The habit grows into a trait, a trait becomes a constant state.
If the brain is out of balance and loses its flexibility, you may get stuck thinking when trying to sleep, dreaming when trying to concentrate, or be saddled with uncomfortable emotions.
The brain has been pushed out of balance, and has lost the flexibility needed to shift out of it.