Why does Eddie sign up to go to the war?

Eddie signed up to go to war in the novel for a couple of reasons, but mostly trivially and insubstantially. While the war is a major event that impacted his own life and the lives of many others with whom he would interact, he signed up without much thought.
While his mother was not interested in his commitment to the army, his father seems unconcerned, which contributes to his decision to go. Eddie thinks that, as most everyone else his age has done, he should sign up to go to war and not be left in America alone, and left as someone who is "cowardly" by not going. He also always had a desire to "fight the good fight" and defeat bad guys, which, in reflection, is a very childish reason to commit to going to war.

According to the text, Eddie signs up to go to war for two reasons. First, he feels that it is expected of him. After all, many young men his age have already signed up. Eddie is convinced that war is "his call to manhood." Second, Eddie has always liked the idea of "fighting the bad guys" and "saving the world."
The text tells us that Eddie's mother does not want him to go to war. Meanwhile, Eddie's father is matter-of-fact about his son's enlistment. Because he has never fired a real rifle, Eddie prepares for war by practicing at the shooting arcade at Ruby Pier. For a nickel, the machine at the pier lets Eddie practice shooting metal slugs at pictures of jungle animals.
For his part, Eddie knows that his work as a maintenance man at Ruby Pier will soon come to an end. He notes that the war has caused pier business to drop. Only mothers with their children frequent the park now. The men have left to go to war. Eddie spends his last nights practicing on the arcade rifle.