Yes, negative thoughts can cause depressed moods. But we cannot oversimplify. Depression also causes negative thoughts.
The best approach to a clinical depression is to tackle the medical angle in cooperation with your doctors (you get a say in treatment, too, that is critically important), and then immediately begin training the mind to think positively as a habit. But the chemicals themselves can keep you thinking negative thoughts just as surely as a brick wall can keep you from walking through it.
Never linger too long with the ignorant,
throw stones at their talk.
Walk only with the lovers,
the mirror of the soul gets rusty when
dipped in muddy water.