You are rolling along, and things are OK, and then BAM - everything tastes like a penny, looks like a pile of Sunday night homework, sounds like soulless pop...
Depression is sneaky because it disguises itself, altering your perception immediately, making you feel that this state of bleak is normal...and having that feeling that your state of normal is bleak feeds into a dangerous downward cycle towards a more robust depression.
I hate feeling this way; I like to believe that I have achieved a heightened consciousness of self that allows me to recognize my own depression (to see through that disguise) pretty quickly, but that belief is likely delusional.
In Terry Pratchett's "Discworld" books, and specifically those dealing with the witches, he introduces this concept of "third thoughts" and "fourth thoughts" - the general idea is that everyone is capable of having "second thoughts" wherein they apply a basic level of analysis to their own thinking, but that sometimes it may be valuable to analyze your analysis. The recursive nature of this analysis brings a danger of paralysis, and so must be applied judiciously. I'm no master of the judicious application of self-analysis, but I would like to believe that such analysis has added value a time or two.