There's another side to being optimistic that's extremely important when you're proposing an idea. I don't mean assuming every new idea is great or that every proposal will work out exactly as planned; that's obviously a recipe for disaster. In my experience, any sufficiently technical person will already have an opinion on whether or not some proposal works by the instant someone finishes explaining his or her big new idea. A lot of times, these opinions will be correct. However, often these opinions will be based on assumptions from situations in the past that were similar in some way but aren't necessarily important now.
As someone proposing a new idea, you should find out what the assumptions behind these objections are and why they're considered relevant now. Make someone explain the fundamentals of why something won't work instead of immediately walking away and telling yourself, "oh, someone objected, clearly they know more than I do and this idea will never be good."
Getting into the habit of identifying your assumptions and challenging them also helps you keep a more objective viewpoint over time by helping you to understand accepted wisdom instead of simply parroting it.
As someone proposing a new idea, you should find out what the assumptions behind these objections are and why they're considered relevant now. Make someone explain the fundamentals of why something won't work instead of immediately walking away and telling yourself, "oh, someone objected, clearly they know more than I do and this idea will never be good."
Getting into the habit of identifying your assumptions and challenging them also helps you keep a more objective viewpoint over time by helping you to understand accepted wisdom instead of simply parroting it.