When a bloody and costly war takes place, it is human nature to look for someone to blame for how things went wrong. Countries believe they are right in their aims in fighting a war, and they believe the enemy nation(s) they fight against are wrong. Typically, those who win a war will feel their cause is justified and will seek to cast blame on the losing side for the devastation caused by the war. European leaders of the Allies felt that the Central Powers were responsible for causing the war, and Germany in particular as the leader of the Central Powers. So they sought to punish Germany, making Germany admit to "war guilt" and imposing heavy financial reparations and other restrictions on Germany.
In retrospect, this seemed very much the wrong approach because it led to a further and worse conflict, World War II. Assigning blame to Germany for causing the war also failed to recognize the fault in all the nations of Europe for creating the situations that led to that war. Woodrow Wilson's ideas (outlined in the 14 Points) might have done more to rebuild a peaceful Europe after the war.