Final answer:
A country may switch from a common currency to its own to regain control over its monetary policy and address its unique economic needs or due to political shifts valuing national sovereignty.
Step-by-step explanation:
The decision for a country to switch from a common currency like the euro back to its own currency can be influenced by multiple factors. A merged currency, while eliminating foreign exchange risk, means sacrificing control over domestic monetary policy to a central authority, which may make decisions that are not in alignment with a country's specific economic needs. For instance, member nations of the Eurozone are subject to the monetary policy set by the European Central Bank, which may not always coincide with the individual economic situations of countries like Portugal or Greece. Factors that might compel a country to revert to its own currency include having greater control over monetary policy, addressing national economic crises, or political shifts that prioritize national sovereignty over economic integration.