I think the critical distinction to make is that if I trade you fish for wood, we are both getting something we need more than what we have. But if I trade two

I think the critical distinction to make is that if I trade you fish for wood, we are both getting something we need more than what we have. But if I trade two securities with you, we are essentially making a bet- and with a bet, one party wins and the other loses. It's a truly zero sum game.