Answer :

iloveonedirection
World War I:
One reason the US joined WWI was because of its allies. The US was allies with Great Britain, France, etc, and the job of an ally is to help each other out. Also, the US was shipping goods over to Great Britain and the Germans caught them. As one of the boats sailed over to Great Britain, the German U-Boats sunk it, causing anger. This was the main reason the US joined the war - they were angry at the Germans for sinking a passenger ship. 

World War II:
The US entered WWII when the Japanese bombed Pearl Harbor. Before that, they had stayed neutral, however, once US land was attacked and bombed, they decided it was not okay and they needed to join the fight. This was actually 2 years after the war had started. The reason Pearl Harbor was such a major bombing was because that was a huge naval base and it wiped out a lot of the American Navy.
ampritchard
One reason why the USA joined in WW1 was because of the sinking of the Lusitania. It was sunk by German Subs. There were many Americans on that British ship. Another reason was the Zimmerman Telegram. The Germans cozied up to the Mexican government. They promised the Mexican government that if the USA got involved in the war then they would arm the Mexican government so they could take back the territory the Mexicans lost during the Mexican-American War. These are the two major reasons why the US got involved in World War One.
The USA became involved in World War Two because the Japanese  bombed Pearl Harbor.  In 1941 Pearl Harbor was his by Japanese fighter planes. Once the US declared war on Japan, Germany and Italy declared war on the US.  Hopefully this helps.