Hm, well, this thread doesn't appear on the "ask" section of HN, only in "new", so I think I may have missed something if there's a way to control that.

Rationality can definitely lead to pessimism if you're not careful, but I think most pessimism is actually quite irrational. Rationality can also be used to support optimism (for example, it's rationally possible that I become really successful). So I try to live by what I call "optimistic rationality" (if that hasn't been coined already. :) )