A reasonable question, to which several objections have been raised. There is the bad precedent set by negotiating "under the gun", and the denial of the right of sovereign states to determine their own security arrangements. Furthermore it is ludicrous to suggest that NATO, which has substantially reduced its forces since the end of the Cold War, could be perceived by Russia as a genuine threat of aggression.
But apart from these points, excluding Ukraine from NATO is just one of a number of Russian demands, set out in a Russian Federation proposal for a Russia/NATO agreement put forward on 21 December 2021.
This includes a provision that NATO forces be excluded from all countries that joined after 17 May 1997 (the date of an agreement between the Yeltsin government and NATO). The effect of this would be to exclude NATO forces and weaponry from all of the states of the former Warsaw Pact, (Poland, Hungary, the Czech Republic, Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, Slovenia) leaving them defenceless.
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Article 4
The Russian Federation and all the Parties that were member States of the North Atlantic Treaty Organization as of 27 May 1997, respectively, shall not deploy military forces and weaponry on the territory of any of the other States in Europe in addition to the forces stationed on that territory as of 27 May 1997. With the consent of all the Parties such deployments can take place in exceptional cases to eliminate a threat to security of one or more Parties.