Why is South China Sea a very big issue?

Background of South China Sea conflict
In the past 20 years right before the WW2, Japan was reigning South China Sea. However, after being defeated in the war, they stepped back from the territory. About 6 countries started claiming for sovereignty over the Sea. The countries, which are involved in the territorial dispute, are Vietnam, Philippines, Taiwan, Malaysia, Brunei, and most importantly China.

Why is South China Sea important?
1. The South China Sea is a region of tremendous economic and geo-strategic importance. One-third of the world's maritime shipping passes through it, carrying over USD 3 trillion in trade each year including imported crude oil. About 80 percent of China's energy imports and 39.5 percent of China's total trade passes through the Sea.

2. Huge oil(estimated about 11 billion barrels) and natural gas reserves are believed to lie beneath its seabed. The disputes include the islands, reefs, banks, and other features of the South China Sea, including the Spratly Islands, Paracel Islands, Scarborough Shoal, and various boundaries in the Gulf of Tonkin.

Why is U.S. involved in this conflict?
There are four islands in the South China Sea. Spratly Islands and the Paracel Islands are the main issue among those four. Vietnam and Philippines challenged against China for the territory during 1974-1999, but failed to conquer. In 2013, China built military bases by building artificial islands which later created complaints from neighboring countries about 'Navigation Freedom'.

Experts now see the South China Sea as a conflict between U.S. and China, and forecast that there is high possibility to escalate into a military conflict. Chinese artificial islands have latest missile in that Sea; however, more than half of the U.S. forces in the Indo-Pacific region are near there. If the conflict rises between U.S. and China, then the countries nearby will take the risk of damage.

Source: Wikipedia about South China Sea
https://en.wikipedia.org/wiki/Territorial_disputes_in_the_South_China_Sea