Why did the united states decide to enter the war and fight on the side of the allies?

I would have to ask you if you mean WWI or WWII?
For WWI, the US entered the war because of the sinking of the Lusitania by German u-boats and the Zimmerman telegram from which Great Britain intercepted from Germany to Mexico saying that if it joined the war on Germany's side they could get part of the US when they won the war.
For WWII, the US entered the war after the bombing of Pearl Harbor by Japanese forces.
Both times the US joined on the side of the allies because those were their allies at the time before the war and because it had been attacked or threatened by the other side which prompted the US to declare war on them leaving it on the side of the allies either it liked it or not.