I've stated the reasoning already: in an economic activity where one needs to loose for the other to win value is not created, it is being transferred from one hand to the other, net result to the overall economy is 0. I give you 100$ bill, you give 100$ to your friend, he gives it back to me, no value was added to the overall economy.
Now you can say that exchanges have positive externalities and you would be correct. But you'd have to analyze those externalities to have an opinion.

Your turn.