Final answer:
Countries wanted to join the European Union to foster economic growth, political stability, and to avoid conflict by linking their economies, introducing a common currency, and removing barriers to trade and movement within Europe.
Step-by-step explanation:
The main reason for countries to want to join the European Union after its initial success was to develop environmental, political, and economic agreements within Europe that would foster closer integration, economic growth, and stability. The European Union aimed to avoid the devastation of war through closer economic ties, and this began with a free trade association, which eventually evolved into a common market, and ultimately transformed into a full economic union. One of the key goals of the EU included the introduction of a common currency, the euro, which helped phase out most of the former national forms of money and was a move towards monetary unity. Another central objective was to eliminate barriers to the mobility of goods, labor, and capital across Europe, fostering a more integrated and efficient European market.