Why is NATO in Denial of West's Non-Expansion Vows & Importance of Russia's 'Sphere of Influence'?

In response to Russia's security proposal envisaging NATO's non-expansion to the East the alliance's secretary-general Jens Stoltenberg claimed that the military bloc had never vowed not to enlarge, citing its founding documents and legal binding treaties. International observers have discussed Stoltenberg's stance and reasoning.