One very good and possible reason (that was at the same time very logical from his perspective) why George Washington decided to call for a policy of neutrality was to avoid United States participation in European wars (2). And he did this with a good reason. A young nation as the US in those times couldn't afford making and drafting men into an army and then shipping them to Europe to help a possible ally, they needed to build their own nation at the time.
The idea of Manifest Destiny changed insofar that the United States now believed that they should invade other countries that didn't have democracies and had religious prosecution and things like that and that they should help end it and make the people of those country as free as those in the US were.
