Since the State of the Union address, especially his back-and-forth with House Republicans, President Obama has focused a great deal of attention on Republican obstructionism. These two posts from Jim Fallows have gotten some attention, including from me. But the topic needs to be considered with a bit more precision. First, a great deal of Republican opposition stems from simple ideological disagreement. Some of that ideological disagreement is couched in disingenuous language -- i.e., Republicans claim to favor covering the uninsured but oppose any plausible mechanism to achieve it -- but it is ideological disagreement at root. You can't fault a party for obstructing ideas they disagree with. You can fault the ideology that causes the party to disagree, but not the obstructionism that is the natural product of that disagreement.
Second, Republicans have opposed some policies they previously favored and, indeed, crusaded for, such as a national debt commission. It's rather galling, and the reply by liberals and even some non-liberals (see this Fred Hiatt op-ed) has been indignant. But it's important to keep in mind the larger point, which is that the system rewards such behavior. If you're in the minority, your surest path to regaining power is for the majority to fail. As long as that's the case, we won't be able to count on some code of chivalry forcing the minority party to act against its interests.
Ezra Klein has put this in the most succinct way: "The government can function if the minority party has either the incentive to make the majority fail or the power to make the majority fail. It cannot function if it has both." Pondering the motives of the minority is, ultimately, a distraction.