NATO doesn’t expand on its own. Sovereign nations seek to join for their own reasons. As a sovereign nation, Ukraine had the ability to seek membership.
Ukraine didn't have the ability to join NATO, it's outlined in its founding documents (Constitution, and the Declaration on the State Sovereignty) that derive from declarations signed between USSR and Ukrainian SSR in 1990 ((Chapter IX. EXTERNAL AND INTERNAL SECURITY): "The Ukrainian SSR solemnly declares its intention to become a permanently neutral state in the future, which does not participate in military blocs and adheres to three non-nuclear principles: not to accept, not to produce, and not to acquire nuclear weapons." [1]
