As well as Antonio Damasio points out that feelings are there to modulate our behaviour for our survival, Jonathan Rottenberg says that the mood is there for the same reason (in fact!). Also very little living organisms experience low mood and that is functional for instance to avoid poisoned food. So one of the various characteristics of the negative mood is that it is an adaptation useful for our survival. In accordance with Gestalt Therapy low mood is a creative adjustments that initially is useful for surviving in our enviroment and depression is its pathological fixation.
Another interesting link is that Gestalt Therapy say that the psychopathology is part of our environment and Prof. Rottenberg points out that evolutionary changes in our life style and environment are critical for our experience of low mood and depression.
One of this crucial aspects is that the entire range of emotions are important for our evolution, as also Damasio says we feel emotions for our survival. We cannot be happy all the time, we are not made to be happy but our natural aim is to reproduce and ensure our specie' survival. Now the fact is that in our modern life we want our evolutionary goal to be happyness and our main questions are "Why am I not happy?", "Why am I not happier?", "How can I be happy?".
Lastly, Peter talks about guilt, shame and anxiety in childhood related to aggression inhibition and other interesting aspects are brought to attention, thus I invite you to listen to the whole interview and also to have a look at the Gestalt Therapy perspective on depression reading Jan Roubal article (see links below). Enjoy and enrich yourself!