I have a couple of problems with what you say...

First, what in practical terms do you suggest? Our nation has been a two party system from the start with almost no major influence from third+ parties. Why? Two reasons, I think. One is the winner-takes-all system rather than proportional representation. It would take a Constitutional Amendment to change that and it is hard for me to see that having much of a chance of flying. Second, our nation has at the core of its political thinking two inherent, related dichotomies: states-rights vs. Federalism and loose vs. strict interpretation of the Constitution. In general you get these two dichotomies defining in some way the two parties. These two dichotomies were left in place by those who established our system of government I think partly intentionally because the balance between the extremes of these dichotomies works better than either in isolation, and because they themselves were so split over them that they couldn't resolve them but could only compromise. I may not like the 2-party system per se, but I do think this odd balance between ends of two dichotomies works better than dominance of any extreme...and, arguably, some of America's worst moments come when on extreme dominates. Interestingly, ancient Greek political thinking also saw the balance of two extreme systems ideal, and often they two saw ongoing infighting between two factions.

So our system is a balanced system that tends to split into two camps, and this reinforces the basic structure of the system that favors a party in presidential power and one opposition party. I have heard no real workable suggestions that seem likely to change this with the possible exception of instant runoff voting that might in rare cases give a better chance to a minor party.

Which leads to my second objection to your portrayal of our system, though this is a more fuzzy one. Given that we are by inclination and bu structure a 2-party system with little chance of majorly changing that, I think it is extremely important to recognize that one of those two parties is far more of a coalition of interests than the other. The modern Republican party tends to be narrowly focused, representing a more narrow slice of America. That means overall there are more registered Democrats than Republicans, but Republicans are far more loyal to their party and far more consistent voters. The modern Democratic Party (and to some degree the entire history of the Democratic Party, more or less) has always been more a loose coalition representing farmers, labor, etc. Hence the famous Mark Twain quote, which most Democrats today can sympathize with. And hence the more recent view that the Democrats have primarily become a coalition of the competing interests of blacks, Jews and Catholics. That of course is a gross oversimplification, but for some time, and to some degree even today, those three very different and competing interests are the most loyal Democratic voters.

And I guess a final objection I have to what you are saying amounts to asking just what about human nature suggests to you that partisanship is anything but how human nature works. In all of history I can't really name any major culture that either wasn't rife with partisanship or was oppressively dominated by one side of a previous partisan battle. The best I see happening over the entire expanse of human history is a system that by and large channels the human reflex to define the world as "us" vs. "them" into the most constructive and least destructive pathways. And which best allows cooperation and power sharing between the us and them sides.

So, given our two party system, which I don't see changing any time soon, and given what seems to be inhernent in human nature, we are lucky to have one party that already represents a vague and shifting consensus of competing opinions because it represents a larger slice of American voters and is more open to pretty much anyone. I kind of LIKE being a part of the party that at the same time included both Zell Miller (yuck!) and Barney Frank among its members. Now I really don't regret that Miller is gone. But I am willing to accept Lieberman in the same party I accept Barney Frank.

We could all grow old and accomplish nothing if we wait for our system to change and become something other than a fiercely contested, two party system. For my part I am not willing to wait. So instead I pick the party that comes closest to my ideals and work with them. More often than not, I am proud to call myself a Democrat because I genuinely LIKE what most Democrats represent most of the time, though I think the inherently vague and shifting consensus that is the Democratic party makes it slightly less reliable than I would like in regards to my personal political preferences. And I don't really feel any need to apologize for my being proud to be a Democrat.

This is the system we have and which I am willing to work within because that is how I see things getting done. Right now things ARE getting done after years of mismanagement by a more narrow political establishment. And I feel good about that.