It seems like the public welcomed the entry and there were more pro-war sentiments than anti-war. From what I've read the public felt that it was their duty to help the country. This seems to contradict the general notion that US was isolationist pre-WWI. But why did the public welcome US entry? If I recall correctly there were 13 million men who reported for 'selective service' and plenty were happy to join. Women even shamed those who didn't go. What was the reason for this?
There was in fact a gradual shift in US public opinion away from isolationism prior to 1917. The central issue was Germany's unrestricted submarine warfare policy, which often indiscriminately destroyed both British and neutral shipping that resulted in the death of American citizens; demonstrated most prominently by the sinking of the Lusitania. While Germany - rightly - noted that this was simply in response to British blockade, Germany failed to realize that there was a gulf of difference between the German and British methods. For the Americans, their quarrel with the British blockade was merely a question of commerce. By contrast their quarrel with the Germans was a matter of life-and-death for American citizens traveling on ships plying the North Atlantic.
What really tipped the scales moreover was the Zimmerman Telegram, wherein Germany promised to support Mexico if it went to war against the United States. This pretty much solidified support for the war.
^ these answers should be pertinent.
/u/Zinegata pretty much hit the nail on the head so to speak, so I'll simply add on to what they've said. The submarines and the Zimmerman Telegram were an immediate threat to the United States, while the Belgian atrocities, the Lusitania sinking, the deportations of Belgian labourers, and the scorched earth retreat to the Hindenburg Line had all served to destroy German credibility in the eyes of the American public.
More so, the way the British conducted the Blockade was in great contrast to the Germans submarine efforts. The Blockade was clearly defined, based in the mouth of the Channel and between Scotland and Norway, with British ships carrying out Patrols and serving in recce lines. Above all, British methods detained ships and removed any contraband, but the ship owners were compensated and there were regular/semi-regular negotiations with neutral country's representatives. The Germans on the other hand, simply carved out a large swath of the Atlantic as a war zone, and sent out U-boats to sink anything they pleased. As Gordon Corrigan put it, The British stopped the ships and searched for contraband; the Germans sank the ships and drowned their crews.