The official excuse given was that Germany by declaring 'unrestricted' submarine warfare was in fact already waging war against the US of A and so what the Heck might as well make it official. There was also an argument voiced that Germany was encouraging Mexico to take action against the US of A.
The truth is that it was mostly in the best interests of the US of A to get involved. On the grounds that Belgium was an innocent invaded by a thuggish neighboor (think Kuwait) and that Britain and France were 'democratic', it only made sence for the US of A to be the 'arsenal' of democracy. Since the war was being fought in Europe no American territory was threatened and also the American military could gain valuable war experience on real actual battlefields.
In the end the US of A emerged with enhanced prestige and as a major player on the International scene. And American agriculture & industry benefitted enormously.