Well, there was the Zimmermann telegram, intercepted by
Royal Navy cryptanalysts in London, which promised Mexico
its lost territories back in exchange for a pact with
Germany.
There was also the resumption of unrestricted submarine
warfare which resulted in the sinking of US vessels
in the "war zone" declared by Gerrmany.
I believe these were the reasons cited by President Wilson.
There also was a popular fervor resulting from the lurid
Allied propaganda of the time.
Our nation was suckered into WW1 as it was suckered into the Spanish American war.