Back when he was president, Barack Obama told me that only two people treated any interaction with him as a zero-sum game. One was Vladimir Putin, the other congressional Republicans. Both behaved as if there was no such thing as a win-win situation: Any gain for Obama was a loss for them, and any gain for them must also entail a loss for Obama. The moment that the Russian president or congressional Republicans saw he wanted something, they went to work trying to keep him from getting it -- even if it was something they might otherwise have approved of.
Approaching any aspect of life as a zero-sum game has obvious practical costs: Deals that leave some people better off without making anyone else worse off suddenly don’t get done, because making some people better off now, by definition, makes other people worse off. It also comes with some psychological side effects. It cripples your imagination. It blinds and deafens you, as you sort of know what your adversary is going to do or say before they do or say it. Or, rather, you know how you are going to make sense of it: uncharitably.