Why the government is shut down?

In democracy majority rules. However, it is unwise to implement a major law with a narrow majority, and despite the strong disapproval of the main opposition. This is how the Affordable Care Act, aka Obamacare was voted in. Then it won a narrow support in the 2012 election. Slightly more than 50% of Americans are for Obamacare, and slightly less than 50% of Americans are strongly against it. The country cannot function when a narrow majority forces its policy on the meaningful minority.