It has been estimated that we have anywhere from 25,000 to 50,000 thoughts a day. If your cast of mind is predominantly negative, imagine how many negative thoughts you are generating daily—thousands upon thousands. That is precisely the case with
depression.
One of the features of depression is pessimistic thinking. The
negative thinking is actually the depression speaking. It's what depression sounds like. Depression in fact manifests in negative thinking before it creates negative affect.
Most depressed people are not aware that the despair and hopelessness they feel are flowing from their negative thoughts. Thoughts are mistakenly seen as privileged, occupying a rarefied territory, immune to being affected by mood and feelings, and therefore representing some immutable truth.
Compounding the matter is that negative thinking slips into the brain under the radar of conscious awareness and becomes one of the strongest of habit patterns. People generate negative thoughts so automatically they are unaware that it is happening, that it is actually a choice they are making.
https://www.psychologytoday.com/articles/200107/depression-doing-the-thinking