We’ve all felt sad at one point or another but depression is when the sadness is so extreme that it alters daily activities. Depression is a mental illness because it causes one to view the world pessimistically. These people often feel hopeless and lose all self-motivation.
The reason this mental disorder is an internal conflict is because it causes people to question there existence. They are constantly trying to find their purpose and have ill-natured thoughts such as self-inflicted pain and/or suicide.
Realistically, every internal thought is a conflict. Decisions are made through our internal conflict. Mental disorders harm our natural, healthy thoughts and can cause abnormal amounts of stress.