It is just an excuse. If the country wants to invade another country, they will always find a good excuse. This is not only about Russia - take any recent war US took part in or any other war. Nobody wants to look evil.
Regarding NATO expansion, of course it serves the interests of USA but it can be also good for other countries. For example, the presence of NATO troops in Baltic countries helps them to stay independent and avoid repeating the situation that has happened in 1939 [1].
[1] https://en.wikipedia.org/wiki/Occupation_of_the_Baltic_state...