It is the stock in trade of military professionals to look first at strategic capabilities and trends, rather than declarations of intent. So it should not surprise anyone that the creep of a great and exclusive military organization toward Russia's borders is of concern to the Russian military. It would also seem unavoidable that any Russian politician hoping to keep his or her position would express concern. This concern need not focus principally on the unlikely prospect that NATO might someday take an aggressive turn. More to the point is the effect military power has on politics every day.
Nations routinely have differences of interest and perspective.
The settlement of these differences need not involve conflict but they always involve calculations of power and position. NATO expansion diminishes Russia politically and does so at a time when Russia is already weakened, fairly accommodating, and facing great instability at home and on its southern borders. Moreover, third parties are watching, and some of these directly and immediately engage important Russian interests. Russia is naturally concerned about the tens of millions of Russians living outside Russia as minorities in other former-Soviet republics. So NATO expansion effectively squeezes Russia between a rock and a hard place.