If NATO was nothing but a self defense pact, I'd tend to agree, but then if that was the case no one in NATO would want to expand too much.
NATO is not just a self defence organisation. It's an offensive tool. NATO has gone into more offensive missions (at least 3) than truly defensive missions (literally zero - I don't count the invasion and occupation of Afghanistan as a defensive act).
For many members of NATO, the only existing military threat is within NATO itself (see: Turkey and some of its neighbors).
The truth is, NATO expanding is a great way for the West to force it's enemies to increase their defense spending and to lower the cost of offensive military interventions.
