As I wonder why this is, I think it comes down to two or three basic causes. The first is that as I learn more and more about the dark stains on American history - for example the deliberate extermination of Native Americans (George Washington himself ordered the genocide of the Iroquois people), the enslavement of African Americans, the outright imperialistic aggression of the US during the Mexican-American War and the Spanish-American War (and not to mention our current war in Iraq) while at the same time justifying this aggression through the language of freedom or security or Manifest Destiny, the internment of Asian Americans in prison camps during WWII, the puppet dictatorships we've helped establish in Latin America, etc. - the less I can believe that America is significantly more benevolent or altruistic in it's motives than any other nation in the history of humanity. We seem to be just as a self-serving, just as hungry for power, wealth and empire, and just as willing to use unethical means to achieve our ends as most other modern nations. Again, don't get me wrong, I don't think America is the worst nation ever, and I'm grateful for the many things we have gotten right over the years (e.g. civil rights, universal suffrage, pro-labor laws, the basic freedoms in our Bill of Rights, etc.), it's just that after having visited and/or heard about many other great nations from people who are just as proud of their homelands as Americans are of theirs, I'm less inclined to feel that America is in some way better or more worthy of my respect than any of these others (And why should we look at it as some kind of competition anyway? By whose standards are we to judge which is the "greatest" nation?)
And closely related to this insight that America is really not much different in it's motives or means than most other human governments is my belief, based on biblical teachings, that all human governments are, by their very natures, what Paul calls "powers and principalities" that are culprits in the systemic oppression of the poor and the powerles. They are all, in the symbolic language of John's Revelation, Babylon, and while America may be the best Babylon that has yet existed in the history of the world, it is still Babylon, and as such is fundmentally opposed to the reality of the Kingdom of God.
And that I guess is my first and foremost reason for not feeling very patriotic towards America, because I have an primary alliegance to a much higher national identity, that is, the Kingdom of God. As Peter says in his first epistle: "You are a chosen people, a royal priesthood, a holy nation, a people belonging to God, that you may declare the praises of him who called you out of darkness into his wonderful light." And later he calls us "foreigners and strangers in the world". He commands us to honor and obey earthly authorities, but no where are we told to confuse their reign with the reign of God over the whole earth. This is why I get so frustrated by churches who incorporate hymns to America and patriotic symbols into their worship, it seems too much like idolatry. It seems like we end up with a syncretistic blend of civil religion with Christian faith. And when that happens, then too often I notice that our commitment to America starts to override our commitment to some of Christ's explicit commands - such as when our desire for "security" and "freedom" overrides Christ's command to love our enemies; or when our commitment to American free market capitalism overrides Christ's command to serve the poor directly and tangibly and not simply through the "trickle-down" charity of "stimulating the economy" by being good materialistic consumers.
Anyhow I guess my point is that because my wholehearted alliegance is pledged to Christ and His Kingdom, I don't have any left over for a mere political nation state. I will do whatever I can to help America truly reflect and promote the values of God's Kingdom whenever possible, but never will I confuse the fact that my true citizenship and true loyalty lies with that Kingdom and not with the kingdom of America.
But I guess that maybe if you define patriotism in that sense, i.e. in terms of wanting to help America become better than it actually is, to help America truly bless God (rather than asking God to unconditionally bless America) then perhaps you could say I'm patriotic after all.