Is being an eternal optimist a good thing or a bad thing?

I would probably describe myself as optimistic but really I'm not - I'm a gloomy guts with a strong survival instinct which is something very different. So really it's an animal instinct forcing me to plod on and not give up, but I moan and whinge the whole time about the small stuff in life. Put a gun to my head though and I turn into an over enthusiastic girl guide.