One of the reasons to join NATO was that Russia (Soviet Union) had a history of reverting political changes in Eastern Europe; see Prague Spring. Given this history, a political change from communism to democracy seemed half-assed without also joining NATO, because it seemed like a question of time until someone in Russia decides "okay guys, your political regime will now change to this" and send tanks to enforce the decision.

It felt like without NATO, the future of Eastern Europe would be decided in two steps. 1) Russia will decide what political regime it wants, which may take a few years, but when the decision is made, then 2) the tanks will come and enforce the same regime in other countries. So, unless you agree that this is how the political regime in your country should be decided, the only safe alternative is to join NATO.