The highest proof of George Washington's Virtue

I've been reading a biography of George Washington. One of the most amazing things about Washington was that he stepped down after he won the revolutionary war. His troops adored him and the whole country idolized him. He could have easily became king of the United States. This stepping down from power, caused more admiration from Europe than his conduct of the war. Chernow writes:

When John Trumbull later painted a series of portraits for the U. S. Capitol, he chose Washington's resignation at Annapolis as one of the crowning moments of the founding era and the highest proof of Washington's virtue. At the time of the resignation, Trumbull was in London and recorded European wonderment at the news, saying that it "excites the astonishment and admiration of this part of the world." (p. 457, "Washington: A Life" by Ron Chernow)