Why did americans change their mind about entering the war after the attack on pearl harbor?

Because before Pearl Harbor the United States was relatively peaceful with the Axis nations of Germany, Italy and Japan, and still had their representatives in Washington. And wanted to remain so, and stated it publicly. So Japan betrayed us by trying to act like we can seek peace. Now, the US government was always leaning towards the Allies and helped them and supplied them with war material before Pearl Harbor was attacked. All we needed was a reason to unite the American people with a reason.

When someone attacks your family you want to defend them to the utmost possibility, it's the same way when someone attacks your country, we were forced into the war, and even when they attacked all we did was declare war on Japan, Germany declared war on us and we did after them. So to defend ourselves we had to become 100% committed to stop "The onslaught against us".