I think also Russia doesn't want the headache of defending against a nuclear attack launched from Eastern Ukraine or the headache of worrying whether anti-ballistic missiles stationed in Eastern Ukraine might provide a decisive factor in a neutralization of Russia's ability to launch a nuclear retaliatory strike.
The US really doesn't have to think about these things because there are oceans between it and any potential adversary powerful enough to threaten it: any defense of the US would rely mostly on its navy and air force. In contrast, the most destructive wars are those fought with land forces, e.g., WW I, WW II, the Chinese Civil War and the one time (namely, the US Civil War) that a big land war was fought in North America. (The American Revolutionary War was a mild affair compared to the 4 wars I just listed because unlike the 4 wars I just listed, the homeland of one of the combatants was never in jeopardy.)
The US security establishment is privileged because it does not need to become competent at sharing a land mass (or, worse, a plain such as the European Plain shown in the link below) with nations with high military potential:
https://en.wikipedia.org/wiki/File:European_plain.png
