John Hay sent the Open Door notes to Germany and other countries to propose that all Western countries should be given equal access to China. The US wanted to be able to trade with China on an equal basis with the European countries.
The United States was concerned about this because they European countries were starting to carve China up into what were called "spheres of influence." In each of these, one country dominated trade. The US was worried about getting shut out, so they proposed the idea of the Open Door.
Open door notes refer to the communications sent bu U.S. Secretary of State John Hay during September-November 1899 to France, Germany, Italy, Russia, and United Kingdom, asking them to declare formally that they would maintain Chinese territorial and administrative integrity and would not interfere with free access to the ports in China under their influence.
The need for such notes, which advanced the general open door policy, was felt by USA in view of the increasing influence of European countries and Japan in China and which could result in Division of China among these countries, harming the interests of USA. With the acquisition of Philippines in 1898, USA considered itself as a imperial power in East Asia and wanted to assert its power.
Acquisition of the Philippines gave the United States an opportunity to trade with China. Secretary of State John Hay issued the first Open Door Note in 1899, urging European powers that had carved China into economic "spheres of influence" to respect the idea of fair competition. European nations were unwilling to agree to allow the U.S. open trade with China, but when the U.S. aided in quelling the Chinese nationalist Boxer Rebellion, Hay sent new notes in 1902 simply declaring China open to all nations for trade and commerce.