Public Economic Reasons
1. There is a financial advantage to a reserve currency.
2. Lower borrowing costs.
4. Global financial stability with multiple tent pole currencies. De Gaulle's speech on the USD + gold from '65.
5. Independence from the dollar.
7. Trade and monetary union would prevent wars. >whatever<
"How Dare You Discuss These!" Reasons
1. Banks play financial pairing trade games in a much more liquid + broad market with the new currency.
2. Free movement of low level labor on the same currency basis would destroy earnings of middle and lower classes of wealthier nations, helping capital owners in all nations.
3. Poorer nations can borrow at lower interest rates to consume more products while richer nations see their currency lower in value making exports more competitive. Same with poorer consumers in all nations. Your debt is their asset.
4. Banks could offer policy advice and consulting for a fee as well as sells CDS to manipulate the debt loads of nations in order to meet currency treaty obligations.
5. Backdoor way of imposing supranational authority on sovereign nations first through money, which would cause crisis, followed by fiscal spending unification, followed by consolidated government.
6. The financial and potential government consolidation would act as a counterweight to US dominance making up for the loss of prestige and influence with the destruction of European colonial networks after WW2.
This last one is my focus. For centuries, the world had been under European rule with a European nation's currency as the world reserve currency. Note: some charts show France as the world reserve currency between the Spanish & British. These countries had worldwide empire's. Some even had the arrogant approach that anyone who spoke their language could be considered a countryman (France). Starting with the interwar period and ending with the '60s liberation and independence movements, these nations suffered an immense hit to their status and pride. France was even defeated by the tiny forces of Indochina and had to hand over the job of fighting the Commies to the Americans. The worst part of the post-WW2 era was having to take American aid (Marshall Plan) just to rebuild and suffer under the yoke of the American built institutions like the UN, IMF, World Bank where leadership positions were usually appointed by American politicians. Mon Dieu! Sacre bleu! In pure population numbers, the individual nations could not compete with the USA or USSR, and the Americans took over the global naval power role as well as held air power supremacy. This burned euros.
Look at De Gaulle's biography. The man created Gaullism, which stressed not relying on America but really French power and ambition. This meant nuclear power for energy independence, which was also to fuel their nuclear arms program. France constantly tried to play a triangulated position between the USA and the Commies. It all screams of a mid-life crisis of a declining nation. That is where the euro comes in. It is the financial attempt for independence as well as enhanced status on the world scene because they so desperately wanted to remain a top dog. There is one more event that must have spurred the French to conjure up the euro and an ECB: the Suez Crisis. One of the many methods that different sides deployed to end the crisis, which was basically a 3rd world country and former colonial powers tussling over a nationalized item (the Suez Canal), was Eisenhower instructing the Treasury to sell UK bonds to put the squeeze on the Brits. I wonder if the Chinese do this with the US now. A currency meant independence and global influence. The American could not use money against them as directly as before, and other nations could curry favor by using the euro as a reserve of writing debt in euros. It would act like a national botox injection for declining European countries.
That is truly the heart of the matter. After World War Two and the dismantling of European Empires, these countries sought new roles. The countries were spiritually and emotionally spent by the conflicts. The UK slid searched for decades, and they did find a nice role as the go between for the US and Europe and continued financial power. France went in another direction with it's thorn in the side approach. The euro fits that attitude. The euro was a completely ludicrous idea that had as many holes and negatives as it had positives, and the first recession is proving its folly. It will not survive, and if it does, it will be a rump euro. One last bit that bolsters the ego case. They could have started at any relationship to the dollar. Maybe 2 to 1 or 5 to 1. No. They picked a ratio near parity (1 eur: 1.17 USD). With the swindle of the Europeans complete and destruction of their social welfare systems approaching, individuals (maybe even some economists) may look back and see the euro as the boost of nitrous that set those engines into overdrive. Hurt pride and deflated egos can drive an individual to do rather ridiculous things. Nations are no different.