strong cultural ties to Great Britain
The cultural ties the US has to its former mother country created sympathy in the US for their situation in World War I. Furthermore, the US had a strong trade relationship with Great Britain that was being disrupted by the war.
Wilson, president during WWI, had made it known he favored England. However, with so many immigrants recently arriving in the US from opposing countries, he felt it was best to stay out of the conflict. Investors and businesses continued to trade with Great Britain and their ties to England would eventually bring the US closer to war. Great Britain had become a friend and ally to the US through the 19th century and that sympathy made it easy for the US to choose sides when the time came.