Largely that Europe is relatively small and can be traversed quickly by mechanized armor, as Nazi Germany showed in WW2. The U.S. is worried that an aggressive Russia could quickly roll over Poland, Germany, and France, conquering all of Europe in short order. Having Ukraine be NATO-aligned means that any such war preparations would have to start from Russia itself instead of Ukraine, which adds an additional 500 miles. Crucially, this is just about the combat radius of a modern airplane, which means that if Ukraine is NATO-aligned, a war in Europe could not get Russian air support until they rebuild airfields in conquered territory, while if it is part of Russia, Germany/Italy/Austria/Czechia/Croatia/etc are all within range of Russian airstrikes.
Basically it comes down to mistrust. I suspect that thinking on the Russian side is symmetric, and they don't actually have any plans to attack Western Europe but are worried that a NATO-aligned Ukraine could be a base for Western aggression against Moscow.
...And then that relates to another issue. For the average citizen, rules are rules, borders are borders, and why would you want to attack another country that's minding its own business? For rulers of these countries, rules are just guidelines, and the only thing that matters is power, because how do you think they got to be rulers of those countries in the first place? They're the ones that make the rules, after all, so of course the rules are mutable. And that's why military & political leaders worry about things like balance of power, because at that level power is what determines what does and doesn't happen.