The main motivation that caused the United States to pursue the Open Door policy in China was its desire for economic gain. The American policy was also motivated by a desire for more power.
In the late 1800s, China was being dominated by European countries. Many of them had "concessions" given to them by China. These gave them special rights concerning trade with China.
The United States did not want to be left out of the China trade. Because of this, they advocated an "Open Door" policy stating that trade with China should be open to all countries.