In terms of one's feelings it is natural that Americans would be more global in orientation. America does not have the deep seated historical connection of Europe, Africa or Asia. Many of the Native American died off or their numbers were reduced by disease and history lost. The colonialists and subsequent immigrants all have ties to other areas of the world, and for those who do not know their roots more and more are learning their roots via DNA testing and genealogical studies. Historically, in the beginning we were very divided. The Irish vs Italian; Whites vs Blacks; Quakers vs Puritans -- we were dividend on ethnicity, race, religion, education, gender, etc. However, over time this nation has become more and more united, and to do that we have had to welcome our differences more and more rather isolate ourselves. As such, well welcome other cultures in this country and effectively make them our own (so called melting pot). The reason why expressing an insular nationalism may be viewed with suspicion is because people would be suspect as to what nationalistic trait one would rally around, and would it be used as a pretext toward rallying around prior beliefs which were used to discriminate in the past.
Several of the founding fathers knew we were better off if we choose acceptance of working together/coming together. Thomas Jefferson for example said of his statute for religious freedom in Virginia was “meant to comprehend, within the mantle of its protection, the Jew and the Gentile, the Christian and the Mohammeden, the Hindoo and Infidel of every denomination.” He emphatically wished to avoid a nation dominated by one religion (so called Christian values belief) precisely because he had seen the actions of his own people when we focus on our differences. That is not to say all these leaders all spoke out against gender, race and other discrimination, however, several did. Remember, American Nationalism in the past was a nationalism borne of those in power who dictated what was "American," and in many cases this "American nationalism" meant women, minorities, Catholics, etc were lesser people. Heck, it wasn't that long ago my home state of Massachusetts was excited a Catholic in Kennedy could be elected.