Is pessimism or optimism the most useful starting point for thinking?
I am glad that you can see the link between pessimism and mood. I am definitely not in favour of a whole empty philosophy of happiness but sometimes when I read too much in the direction of nihilism I find that I sink into melancholia. The term is not used that much now, or certainly not within psychiatry. However, going back to my time as a teenager, reading in a library, I can remember reading, 'The Anatomy of Melancholy', by Robert Burton.
I do think that it is easy for a general sense of disappointment in life to overlap with a whole philosophy of pessimism, and it seems that the two can collude together. Generally, I think those who experience more unhappiness are more likely to gravitate towards a pessimistic philosophy. I think it is how far one decides to go, because it can become a downward spiral.
However, perhaps it is also important to acknowledge the pits of despair. One of my favourite quotes from Carl Jung is, 'I live in my deepest hell and from there I cannot fall any further.' One idea which Jung refers to is the idea of enantiodromia by Heraclitus, which refers to the way in which when one of the poles is reached to the extreme, there is often a natural swing to the opposite one. Certainly, I find that if I get into a really negative state, it often seems that at some point, I swing to the a really positive swing. But, I am aware of people who seem to live most of the time in a state of negativity or positivity. I feel that I have a constant battle within myself between a general outlook of pessimism and optimism.