According to cognitive behavioural therapy (CBT), when you're experiencing depression, you will tend to have automatic negative thoughts about yourself, the world and the future. This pattern of negative thinking you deeper into depression. This brings about further negative thoughts; which lead you to feel even more depressed; brining about more negative thoughts; and so on.
Understanding destructive co-dependency. Codependency is a relationship pattern (with anyone) that is self destructive for both parties because we give at our own expense instead of setting healthy boundaries. Read more about what codependency is and how to heal it.