It's always astonishing to me how people here (mostly Americans) basically know nothing about the long history of U.S. proxy wars with Russia (historically USSR) and the long stated desire from U.S. to destroy or regime change the Russian federation.

To answer your question quickly: Ukraine entering NATO constitutes an existential threat to Russia for the same reason as China building military infrastructure in Mexico, Cuba or Canada would pose an existential threat to the U.S. (e.g. Cuban Missile Crisis).

Further reading: https://www.rand.org/pubs/research_reports/RR3063.html