Monroe's greatest asset was his disarming, warm personality. Although conceding that Monroe lacked brilliance and a nimble mind, biographer Harry Ammon pointed out that he had a rare ability of putting men at ease by his courtesy, his lack of condescension, his frankness, and by what his contemporaries looked upon as his essential goodness and kindness of heart. Monroe at least partially overcame an early shyness but remained markedly low-key and reserved, especially among strangers. Acutely sensitive to criticism, he at times took offense where none was intended. Rather than lash back at his critics, however, he usually bottled up his feelings. Operating as he did with such an elevated sense of his own integrity, Ammon has written, he could not easily adjust when old friends failed to approve his conduct.