The problem is that our method of election (winner take all) almost guarantees a two-party system. It's not like people haven't tried (Bull Moose, Progressives, etc.), but when we don't have a parliamentary form of government with proportional representation (like many in Europe), it devolves into us/them parties. Back in college, I remember reading books that proved this out mathematically.
As a result, you get two parties that, in primary season pander to the extremes of their parties (their bases of power), and in general election season run centric, so there is little to differentiate them for voters. Then, the American voter in his/her mind, wanting balance of power, often penalizes a Presidential candidate by taking seats away from his party in mid-term elections, creating a virtual gridlock as long as filibusters are possible in the Senate. Even better, many's the time when a Republican was President and Democrats controlled Congress, or vice versa.
One of the things that interested me the most was that Americans "hate Congress but love their Congressman" as long as he brings the bacon back to the district. Hence, a Congress that is locally responsive but collectively irresponsible.
When you consider the meat grinder one has to go through to get elected, why would anyone want to do it? They may start off with ideals, but soon it all becomes a question of how to survive, until you become a part of the machine you railed against when you started out.
