By the time the United States entered World War I most Americans supported the Allies. There are several reasons for this. One reason was we had more similarities with Great Britain than with the Central Powers. Our language and government were similar to the British language and government. We also had more business connections with Great Britain. We did a lot of trading with them.
There were events during World War I before we joined the war that influenced our feelings. The Germans were violating our rights as a neutral nation by sinking our ships without warning. When Germany agreed to stop this, they eventually went back on their promise.
We also were unhappy that Germany tried to get Mexico to attack us. When the Zimmerman Telegram was made public, Americans were furious that Germany wanted Mexico to attack us. Plus, if Germany won the war, they said they would give Mexico back the land it lost in the Mexican-American War.
The British also used propaganda to sway American opinion. British propaganda was far more effective than German propaganda in swaying American public opinion. There were many factors why we supported the Allies before we joined World War I.