And NATO pushing up to the borders of Russia isn't considered an aggressive move on the part of the USA, because ... ?

Because it wasn't NATO that "pushed up to the borders of Russia" it was the Eastern European countries that fled from Russia into NATO. Not a single NATO tank had to streamroll into those Eastern European countries for them to join . You'll note that none of those nations that joined NATO needed to be invaded and military occupied by NATO -- unlike what Russia is doing now, and unlike what Warsaw Pact did in the past.

Because if any of those countries ask to leave NATO, NATO will leave. However Ukraine, Georgia, Moldova are asking Russia to leave, and Russia isn't leaving.

Because the sovereign and independent Eastern European nations wanted to become part of NATO, and NATO tanks didn't need to force itself on a single nation, it was invited( a single country, nor change the borders, unlike Russia's military occupation of portions of Moldova, Georgia and Ukraine.

Because a NATO country like Poland isn't the one attempting to annex the western portions of Ukraine or Belarus or Russia.

Because when Greece has been recently openly allying itself with Russia, I don't see NATO troops from Italy or Albania or Bulgaria attempting to break apart portions of Greece.

Yay, for simple answers to simple questions.

Americans would feel threatened if say, Mexico joined the Warsaw Pact, no?

Yes, America also often used military imperialism in its attempt to stop Latin American countries from allying themselves with the Soviet Union, (e.g. the invasion at the Bay of Pigs).

So?