Because the House and the Senate failed to come to an agreement on the spending bill which funds the government. The House sent three bills today alone to the Senate which the Democrats there(the Republicans voted for the bills) rejected. At the end of the day when Bohner offered to go into conference to come to a compromise with the Senate, Reid rejected that too. The Democrats have been the one absolutely refusing to negotiate, but have the temerity to blame the Republicans.
2014 fiscal year began on Oct 1, the congress was suppose to pass a federal budget by then. The House republicans tried to pass a budget that stripped funding for the Affordable Healthcare Act better known as Obamacare. The democrat controlled senate refuse to accept this budget because it was in direct violation of the healthcare law. The two parts of congress could not agree on a budget and no budget was passed by Oct 1 causing the government to shutdown. SIMPLE
It was because Obama wanted to have Obamacare funded but the Congress lied to him about not enough money. That and plus Obama was selling illegal weapons to drug cartels and other terrorist organizations.
Basically because Obama has spent too much money and no money was left to pay for government workers. In addition to that, the House voted to defund ObamaCare which would make the money matters so much better, but Reid wouldn't allow the bill to go through the Senate. Pages could be written about the situation, and volumnes when you add all the details. ObamaCare is over 2000 pages long which is the stupid reason they let it go through without even reading it. It's a horrible health care program that would cause the government to be in control of the people. We need free enterprise, not government control. It's communistic.
The government isn't really shut down. Some "nonessential" employees have been given a few days off with pay, but essential government services are still operating.
The "shutdown" is a political stunt to draw attention to government funding problems (of various sorts). It is essentially just the result of multiple disputes between Democrats and Republicans over how to spend government money.