Why so pessimistic?

One can't ignore the presence of a high degree of pessimism in the context of collective action and co-operation, be it Hardin's 'tragedy of the commons', Prisoner's Dilemma or Olson's 'logic of collective action' or even in general perception. The words, 'tragedy' and 'dilemma' for example indicate pessimism.

What gives rise to such pessimism and what effect does this pessimism have on the adoption of collective action and co-operation? And should this pessimism be taken seriously? In the absence of answers to the above questions, more questions arise.

Is the difficulty in achieving collective action and co-operation a plausible reason to be pessimisic about it? If so, can't human nature which is supposed to be responsible for such difficulty be dealt with in a way as to decrease such difficulty (Isn't that the aim of studying CAC?)? Didn't Gandhi do it? By calling him ahead of his times, we discount the possibility of using his ways. Doesn't successful collective action and co-operation give reason to be optimistic because of its very nature, which is quite different from the other ways of doing the same things, for example, through government interventions or the markets which tend to fail in the long run (with questionable success in the short run). Isn't collective action and co-operation, overcoming the acknowledged difficulties, a sustainable way? In this sense, isn't it more reasonable to be optimistic about collective action and co-operation than otherwise?