What are the reasons America entered World War I?

There were a number of reasons for America's entry into World War I—moral, economic, and political. On the economic front, big business in the United States had a large financial stake in an Allied victory. Most of the Allied powers were heavily in debt to American banks and other lenders. If the Allies lost the war, then they would not be able to pay back their loans. By entering the war on the side of the Allies, then, the United States actively ensured the protection of American business interests in Europe.
Anti-German sentiment had been building in the United States for a number of years. However, what really captured the public's imagination more than anything else was the sinking of the Lusitania. Although the British passenger ship was likely carrying military equipment in addition to its human cargo, a large number of civilians—and some Americans—were killed when the ship was torpedoed by a German U-boat.
A further reason behind America's entry into the war was related to the two just mentioned. The Germans were operating an increasingly aggressive and unrestricted campaign of submarine warfare in the Atlantic. Despite promises to stop, the Germans continued to target civilian vessels. Among other things, the Germans' use of submarine warfare damaged United States trade with Europe, preventing American goods from reaching overseas markets.
The infamous Zimmermann telegram was another crucial factor. Zimmermann, the German Foreign Minister, sent an ill-advised telegram to the Mexican government, that suggested if the United States declared war on Germany, then Mexico in turn should declare war on the United States. In return for this, Mexico would receive territory lost to the United States, such as Texas and Arizona. Although the Mexican government had no intention of doing anything so foolish, the publication of the intercepted telegram inflamed American public opinion against the Germans and their allies.