Why do opportunity costs increase as you make more and more butter and fewer guns?

Opportunity costs increase as you produce more butter and fewer guns due to the principle of increasing marginal costs. As resources are reallocated from gun production to butter production, the most efficient resources are used first, and less efficient ones are employed later, leading to greater sacrifices in gun production. This means that each additional unit of butter requires increasingly larger amounts of guns to be forgone, resulting in higher opportunity costs. Thus, the trade-off becomes steeper as production shifts further away from guns.