1. Whenever we have a Democratic president, the Republicans behave as if it was a Communist takeover. They reflexively oppose ANYTHING he says, does, or proposes, in the strongest terms they can muster. They did this with Carter and Clinton too. So about 90% of their opposition to Obama's health care plan is really just opposition to Obama, it's not about health care at all. Republicans have come to see the White House as -their- house, so when a Democrat is elected they feel that a great injustice has been done, that they've been robbed. They don't see a Democratic president as legitimate, and cooperating or agreeing with him, they feel, only hands him a victory and validates his presidency. At this point, all they can think about is to make Obama fail so they can get back into power.
2. The Republicans stand for big business, for profits, return on investment. When a Republican says that we have the 'best healthcare system in the world', what he really means is that our healthcare system produces the biggest profits, creates the most wealth for investors of any in the world. And they're right, it does! Our entire health care system--not just insurance but medical centers, clinics, hospitals, doctors, labs, etc. etc.--is owned by 7 corporations. These corporations are so rich that they are able to spend millions of dollars a year in lobbying and campaign contributions, and traditionally most of this money has gone to Republicans, so which side did you expect the Republicans to be on? 8^)
Sadly, Obama and the Democrats have also gotten a lot of this money, so Obama wasn't able to propose any new plan that cut these corporations out of any of their profits. Obama began his campaign advocating a Single Payer system that works great all over the world, but ended up with a plan that actually ends up being more of a govt. subsidy to the health care industry, and still leaves millions uninsured. So it's really a compromise that -nobody- likes. 8^<