"Inside every cynical person, you'll find a disappointed idealist."--George Carlin
From personal experience, I believe that what happens is eventually the idealists and the optimists get tired of being disappointed. Cynicism is simply a defense mechanism to guard oneself from pain-- more reactionary to an event catalyst, rather than simply "being" on its own.
I think almost everybody, as we all grow into adults and find things not as we wished or imagined them to be, is very suddenly and distressingly disappointed-- with government, with family, with the state of the world, or the very state of the human spirit. And in response to the shock of it, naturally grows a more negative, but also realistic, view of things-- at least as compared to earlier ideals. Like being suddenly submerged in ice water, we tense up, curl inwards, to protect ourselves. And yes, often this does skew into the realm of cynicism, where once the most optimistic seeming to become the most jaded.
However, I have found that in the same regard, as we CONTINUE to grow, we also have the tendency to swing back in the other direction. And not because things have necessarily IMPROVED in any way as we continue to grow, but rather having now had the experience to weigh the good and the bad of humanity, we can then CHOOSE to believe the we are capable of our best.
In this way, cynicism, while a hard and painful stage of life, goads us from the blind faithfulness that is pure optimism, to become the living, acting people needed to change the world. I think, if one is TRUELY an optimist--as in subscribes to the philosophical doctrine wherein one holds the actual world out to be "the best of all worlds," i.e. where one creates the most good out of the least evil--cynicism is not only inevitable, but also a necessary step for us to truly make a difference and create the world we so optimistically believe to exist.