“Depression-related anger can be a reaction to the perceived hopelessness of a situation or even frustration over recurring states of depression [source: MacManamy].
A problem that offers no effective solutions or positive outcomes is going to produce its fair share of frustration and anger. For similar reasons, anger is also a common stage of grief [source: Chakraburtty]. As is the case with depression, grief-induced anger is the result of a feeling of hopelessness and despair.”
Tom Scheve (Howstuffworks.com)