The Chinese government is really trying to start a trade war against the US. China is currently quite bossy and not at all friendly with any country, they are nurturing a conspiracy to dominate the world and this should be suppressed from now on. for this Chinese plan, they were wrong. The United States may be scarce in terms of agricultural products, but it can certainly import produce from other agricultural countries at cheaper prices. Doing so only damages China's economy and they are making the surrounding countries more hostile.
You control food, you control people, you control the energy you control the country, you control currency you control the world, and you control health you control life. Although America is not a large producer of agricultural products, America remains a price maker, not a price taker. America controls and controls the world price of food holdings by intervening in the management of other countries and even by manipulating it.
It's about nationalism and national pride to an extent, but mostly about self-determination. China cannot become a world power if it cannot be the preeminent regional power in Asia. The United States understands this, and this is why we prop up competitors and rivals to China's preeminence. Vietnam, South Korea, Japan, the Philippines. Our military and economic aid to all these countries is designed to keep China from projecting power outside of their immediate sphere of influence. This is also why we keep challenging them in the South China sea. The US is trying to prevent them from challenging US on the global stage by creating regional roadblocks. China wants to overcome those for the same reason - projecting power globally.
As for why, both sides' style of government is a threat to the other. China is authoritarian and keeps power by suppressing freedom and controlling information. An open and free style of government is a threat to those who hold power. In the US, the spread of an authoritarian government to other areas of the globe is a threat to our freedoms. We cannot allow ourselves to be in a position of weakness against a ruthless country that would oppress its own people; what do you think they'd do to foreigners?
In the end, it's all about money, the competition between China and America is heating up because the current Chinese and American leaders are both nationalists. In the previous era, China became America's strategic partner, but in the Trump era, China became America's competitor. Although the ideologies of the two countries are contradicting each other, the current rivalry between China and America began when Xi Jing Ping wanted to terminate the agreement between Reagen and Deng Xiaoping regarding democracy in China. Reagan is a Trump idol. This is what sparked tensions between America and China.
For China itself, China feels that it is a big country as one of the centers of world civilization besides that there have also been bad experiences in the past due to colonialism and a desire to seek revenge. But despite all that, China has plenty of stomachs to feed, so its leaders must ensure food, economic, and political security in order to maintain domestic stability.