Why did Stalin fear a reunited Germany during the cold war?

Before WWII- They knew Germany was angry and wanted revenge also they knew it had some very powerful political leaders who would just love to take over the Earth.

Currently- They are one of the richest countries in Europe and would have no problem manufacturing weapons and vehicles. They also have alot of allies who wouldn't turn their back on Germany. Also they have very advanced technology for a country. As you may of heard German engineering out beats American technology. And finally they would have learned alot from their loss in WWII.