As a matter of fact the optimist is quite optimistic. Nearly always he sees the good part of things. A thunderstorm e.g. is fantastic and interesting rather than irritating or frightening. Getting wet to the core because of bad timing of a bycicle tour (through the rain), makes him laugh rather than shouting curses to the wheathergod. The solution of friends problems is just a matter of different approach and really difficult ones just need some talk to friends, a handful of good will and time to disappear. Own problems get the same treatment, but anyway are so nihilistic compared to problems of others. So why complain? He's had a wonderful live so far and opportunities others have not. Besides the wonderful sun is rising everyday, spending warmth, energy and the most amazing light mankind knows.
And what causes an optimist to get rather unoptimistic? When does the whole live and everything just don't make sense any more? When does even the sun not help any more? It's a thing said to be the most wonderful thing in the world. A thing where no rules or regularities are applicable. (Which is fine.)
The agony of when things related to "the most wonderful thing in the world" go wrong, wash just away future plans, great ideas, enthusiasm, all the optimism...quite everything besides staring...
...for some time.