I think the answer to that is pretty simple: there is a constant swing in support from one party to another as people see (or at least think they see based on what the media is telling them) our country doing poorly. Regardless of uncontrollable factors, people are going to blame the person in charge and look to replace them when they want to see things going better.
As we've all seen (both in history and in the last decade), it's notably easier to mess everything up than it is to fix things. When things are bad, repairs become all the more difficult because they cannot come as fast as people want them. Until we wind up with some sort of utopia where everyone's rights are supported and their freedoms don't encroach upon others (and religious people stop thinking they need the power to control others when no harm is actually coming to anyone) we're going to switch from party to party. The grass is always greener on the other side, and shy of utopia, there will always be another side. So long as things aren't as good as we want them to be, we'll be willing to go somewhere else to help get us a little closer to our ideal.
Democrat, Republican, doesn't matter. U.S. citizens in general will have loyalty to whoever isn't sucking at the time, and that often winds up being whoever isn't in power.
I can agree with pretty much all of this.
Stability can be as much of a threat as instability - the trouble being that the longer we have someone doing damage in a position to do so, the greater the damage can become. The constant switching between parties and presidents keeps us safe from any one person or party molding the U.S. too much into what they want to see it become. Some times this may be for the better, but others it could lead us into ruin. The inherent instability protects us from one single set of views taking control.
I know that stability can also be a problem. Yet then again we also have the power to impeach our president if the country should see fit. One thing I just HATE to see is a president like Bill Clinton pull our country into one of the highest wealth points we have had in like what... 50-60 years? It just doesnt matter though when a jackass can pop into office and destroy all of that. I guess it just baffles me how fast one person can change everything. I mean speaking of leading us to ruin, we did vote Bush two times, and he did serve his full terms. So techinically in my lifetime he has come closer to leading us into ruin than any other former president (my lifetime that is). Sometimes The instability scares me because republicans and democrats vary on so many issues, and when always switching between the two of them... its very hard to find a calm settling point... or to get into the flow of anything for that matter. People do always think the grass is greener on the other side... however sometimes the grass is dead and full of weeds and the "average american" cannot see over the fence until the leap is taken. This posses danger to a country that is very easy manipulated.