Map of US and NATO bases in Europe
NATO's eastward expansion is a process of protecting an ever-expanding territory with ever-smaller forces. Countries that are completely militarily incapacitated join the Alliance, but do not seek any capacity. Moreover, after joining NATO, all countries in Eastern Europe began to reduce their armed forces and military budgets to a minimum, although Brussels and especially Washington wanted the opposite from them. The East decided that it now had no need to strain, because the West, having accepted it into NATO, automatically assumed obligations to protect it. The West wanted the East to make an adequate contribution to the overall military effort, especially since the countries of Western Europe themselves have greatly reduced their armed forces and wanted the Eastern Europeans to replace them "at the battle post". As a result, even from Poland, which actually opened a road to the West for the entire region in the early 80's, the voices of skeptics began to sound. Some Polish politicians, without giving up their Russophobia, started talking about the fact that the Russians at least did not send them to Afghanistan, although they fought there even more actively than the United States is now.
In such circumstances, the official invitation to NATO of Croatia and Albania, and the potential admission of Macedonia, Ukraine, and Georgia there, somewhat resembles a mockery of common sense. A whole crowd of countries with armies that have not even zero, but deeply negative combat capability (this showed, by the way, the fiasco of the Georgian army during the war in South Ossetia) is immediately hung on the block, while the area of responsibility of the Alliance is very significantly expanded.
However, we must not forget about one factor, the most important factor. NATO's eastward expansion is largely an end in itself. It gives the sense of the existence of huge Brussels bureaucracy. This meaning was lost in connection with the collapse of the Warsaw Pact and the USSR, and a new one was never found. Since no bureaucracy in the world will dissolve itself, it will surely come up with a new meaning for its existence and begin to implement it. Extensive expansion of your control area is an ideal solution. Nobody is interested in efficiency. The main thing is accounting and control.