Actually, the first proposals were a bit earlier.
Some milestones can be find there:
en.wikipedia.org
The last straw, as far as I know, was the visit of American B-52s in Ukrainian airspace in the fall of 2021.
Mostly because Russia is a nuclear power, and Ukraine is not. If we want to avoid a nuclear war, then, of course, nuclear powers shouldn't create vital threats (directly or indirectly, nuclear or conventional) to each other.
That's you problem. It's not about territory at all. It's about people. And no, in 2014 Russia didn't take territories from Ukraine. Russia recognized independence of Crimean Republic, and then - rejoint with it. If NATO countries have right to recognize independence of Kosovo, then, of course, Russia have right to recognize independence of Crimean Republic, Donetsk People Republic, Lugansk People Republic, Kherson and Zaporoje regions.
Ask Serbia, Iraq, Syria, Libya, Afghanistan and many other states, invaded by the members of NATO.
Did Serbia, Iraq, Afghanistan, Libya, Syria or anybody else directly attacked any of NATO countries?