Nearly all because the US threaten the South China Sea trade route. Anyone that can't see that the US is the most aggressive force in the world and responsible for more regime changes, war and suffering than any of the "bad guys" isn't looking.
China basically think its their own bit of ocean because it says China in it. Never mind that its 50m off Vietnam's beach and the entire length of Vietnam away from China.
They even found an ancient piece of parchment that says something vague about it belonging to China , which they claim legitimises everything, but doesn't really.
If you have any fancy crockery that gets called "China " watch out , they'll be coming for it.
File:South China Sea claims map.jpg - Wikipedia
en.m.wikipedia.org
Meanwhile the USA are not trying to claim anything or prevent any ships from travelling in that area.
How should Vietnam feel about their Sea Trade Route?
Brunei.
Hmmmm particularly off the coast of Borneo there's oil under them there waves . Black Gold, Texas Tea.