> I wonder, if we could average the feelings of our investing experience across time, would it be averagely positive, or negative? What does your intuition tell you?

Seems like the average would be negative. We feel the lows worse than the highs, so that would tend to pull down the average. 

> understanding our own conditions, motivations, and how we react to different circumstances, because they affect the decisions we make

This is exactly why I tell everyone they should do meditation everyday. I know of nothing else that helps us understand our own mind better than to be alone with it for 20+ minutes a day.

I know, I know, most people aren't going to do that. I find paper trading is a fairly good option for learning to understand how we react to things. But most investors don't want to take the time to do that either. Oh well.