Okay, so you can't stay a thinking being and stay completely out of politics.
Of course, you also can't stay a thinking being and stay completely in either of the two major U.S. parties. They are historical accidents, and don't really mean anything. They are basically random agglomerations of incompatible interests, and precisely two of them exist because you have to get half the votes in the country to win the election. Thus, any interest has to pick one agglomeration or the other, and to live with the bedfellows that chance factors have presented.
Why is that so?
The United States is designed as a duopoly. Actually, it was designed to not have national political parties at all, but the urge to in-group/out-group is too strong in the human species for that to work long.
Notice the format of the election of a president: within each state, in November the voters elect a slate of representatives who will travel to select candidates at the national convention. Only the appallingly bad educational system lets them believe they are actually electing the McCain or the Obama in the November election. In fact, if the electors (ie the slate mentioned above) were to suddenly decide on December 15th that, oh, the Nader should be president , then that is what would happen. Followed by screams of anguish and anger, lawsuits, then the inauguration of the Nader.
Now, you can understand the system based upon the power structures and the communication and travel features of the time it was designed. Thirteen colonies each elected people to go get together and select the best president from the people available. (Well, the best landed white male etc.) The people who were elected - the "electors" of the electoral college, would physically get on their horses or ships and ride to the place where the electoral college would take place, which would take a couple of weeks. When they all got there, they would do their dickering and haggling, vote a couple of times and finally pick a person that the majority of electors agreed on.
Fast forward to, um, television. Let's face it - not one person in ten realizes anymore that they are not directly electing the President. In 1800, no one would have expected a direct election. In fact, they didn't expect direct election for Senators!
Because the Consitution didn't have direct election for Senators. Each state got to decide its own way to pick those guys, and it was usually the state legislatures - the guys in Albany or Austin or Sacramento - who did the picking. That was changed by the Seventeeth Amendment in 1913 to the current direct election method. [Yes, I had to look that up.]
So, along comes television, and people start thinking that they know enough about the candidates to make their own decisions, and that their votes are directly for the candidate. People start selecting the candidates for the parties in the primaries, and the people doing the selecting (ie the primary voters) are now a bunch of folks who, lets face it, don't pay attention until a week before they make their decision.
So you get candidates like John Kerry and George Bush 43.
Okay, pardon me for stating it, but I think if you had an objective qualification scale for who could do the job of president well, out of 400 million people in the United States, does anyone think either of those guys would be in the top, oh, fifty? Nope.
Thousand? Maybe. Certainly, one or both are in the top ten thousand.
The system was designed as a proxy democracy, where you give your vote to a guy you trust to make the decision, and he goes and makes the decision for you. What we have is a kind of drunkard's walk. More about that in some other post.
Back to why we have a two-party system.
Overall, it's simple. You can't become President without a majority of electors. You can't have a majority of electors without being a broad coalition that has a good chance of taking that majority.
Mathematically, there can only be two of those.
More on the practical results of that, later.