I see what you mean but I think you need to clarify your concepts. After you do this you may see things differently. (Note by the way that when people say 'depression' here I imagine that they are talking about 'clinical depression'.)
Not all "mental states" are created equal. For instance you mention arrogance and confidence. These are are attitudes, or personality traits really, not emotions. Love is similarly not strictly an emotion when you think about it, it's a disposition - we fall in and out of and _are_ in love, but we _get_ angry or get sad, we don't _get_ love. Depression, someone mentioned elsewhere is like a filter, or as someone else mentioned is like having a part of you dead inside, the part that makes life bearable, the part that provides resilience to the sufferer against the unforgiving nature of this world we live, joy for life is snuffed out. One gets depressed but it lasts and lasts, anger by contrast is usually fleeting, so are most other emotions.
Depression usually is accompanied by a broken sleep routine, persistent negative thoughts, morbid thoughts, a lack of self-esteem, an inability to remember that life has its ups as well as its downs. Clinical depression wrecks lives. Perhaps "illness" is not the right word but it certainly seems like a profound psychological disturbance, a psychological illness. When we truly have a 100% neuro-scientific of whatever it is we'll be able to use the right terms but for now "illness" may be the best term we have, so we may as well use it.
Not all "mental states" are created equal. For instance you mention arrogance and confidence. These are are attitudes, or personality traits really, not emotions. Love is similarly not strictly an emotion when you think about it, it's a disposition - we fall in and out of and _are_ in love, but we _get_ angry or get sad, we don't _get_ love. Depression, someone mentioned elsewhere is like a filter, or as someone else mentioned is like having a part of you dead inside, the part that makes life bearable, the part that provides resilience to the sufferer against the unforgiving nature of this world we live, joy for life is snuffed out. One gets depressed but it lasts and lasts, anger by contrast is usually fleeting, so are most other emotions.
Depression usually is accompanied by a broken sleep routine, persistent negative thoughts, morbid thoughts, a lack of self-esteem, an inability to remember that life has its ups as well as its downs. Clinical depression wrecks lives. Perhaps "illness" is not the right word but it certainly seems like a profound psychological disturbance, a psychological illness. When we truly have a 100% neuro-scientific of whatever it is we'll be able to use the right terms but for now "illness" may be the best term we have, so we may as well use it.