Well, like it or not, it is one of the primary reasons he's invaded. Its something he's talked about for years...has made it clear he has his own "red line" for NATO expansion, its why he invaded Georgia previously....and has repeatedly railed against expansion of NATO.
I think you need to get over the fact that this is his justification, it just is. He has always viewed NATO as a threat to Russia. You ask, who is he to tell them who can join NATO...he's the guy that wants to reconstitute the USSR, the guy who wants to flex Russia's influence in eastern europe and over their former bloc states, OF COURSE he is going to try and influence who joins an alliance that is primarily in existence to thwart Russian aggression! Come on man!
None of this makes it right, it just is what it is. Thankfully the west stands united against him, NATO will now be stronger than its ever been, and hopefully Ukraine can repel this invasion long enough for the crippling economic sanctions to force Russia to stand down.