Russian security doctrine is based on surrounding themselves with buffer states. Those buffer states cannot be fully sovereign, because a fully sovereign state

Russian security doctrine is based on surrounding themselves with buffer states. Those buffer states cannot be fully sovereign, because a fully sovereign state could choose an alliance with another major power. Ideally they should be vassals, but neutral countries can also be tolerated. This is not an issue if you are far away from Russia, like Germany is. But if you happen to be a country Russia considers to be in their sphere of influence, you may value your sovereignty. Just like Russia has historical reasons to be afraid of an invasion from Europe, countries bordering Russia have historical (and recent) reasons to be afraid of an invasion from Russia. There were some reasons to believe that Russia would change for the better in the 90s and early 2000s. But then Putin took the wrong lessons from what the US and NATO did in Iraq and Kosovo. He learned that if you consider yourself a major power, you are allowed to invade lesser countries as long as you think you have good reasons for it. Then the situation escalated, and now many Russia's neighbors are fundamentally incapable of trusting Russia until there is a regime change or another equally significant change. Germany basically has to make a choice. It can have good relations with Russia or with the countries between them and Russia, but not both. Not until something changes.