It takes optimism to have the guts to dare to make a difference in people's lives. An optimist risks being a fool. A pessimist is never fooled. As long as there are problems, the pessimist feels justified, and there are always problems.
But problems motivate the optimist. Our optimism imposes a duty upon us--do something. A pessimist has no such duty, he has only to heckle. It's a much easer job, being a pessimist. You can do it without getting off your ass.
But you can't be one and be a progressive. Optimism is the entry requirement for progressive thinking. If you can't manage it, you're not a progressive, no matter what your opinions are. You're a kibitzer, a heckler, a Cassandra, and you're no help to the rest of us at all.