Because you don't follow lock step with the agenda of the left. You see, they know what is best for this country, and therefore their actions and policies are designed for what is best for all Americans. They are but humble servants of the people and would never use underhanded tactics to gain power in Washington, or the states. If a democrat is elected, it is simply the will of the people. When republicans win elections, it is only due to sheer ignorance of the voters, or underhanded tactics were used to steal elections.
You are partisan, because you do not recognize this. Prehaps you recognize the failings of the right(so you are almost there) But because you choose to ascribe bad policies and decisions to the left, you must be partisan, because anybody who wasn't partisan would see the wisdom and grace that is a part of liberal politics. And with a black president, your opposition suspicisously not only makes you look like a partisan, but a racist as well. Those two reasons, along with a poor rural education in the south, are the only way anybody could disagree with the policies and the actions of the left.
I hope this helps.