In what sense are the U.S. and China “rivals” today? When can “rivalry” be a good/bad thing?

The United States and China are rivals in the sense that they both claim to have vitally important stakes in the Asian-Pacific region, increasingly bringing the two nations closer to a major confrontation. The United States is a “Pacific nation.” It has a long coastline along the Pacific Ocean; its 50th state (Hawaii) sits in the middle of that expansive body of water; it maintains important alliances with Asian countries, and a large percentage of American trade—more than a quarter—is with Asian nations. U.S. alliances with Japan, Thailand, and the Philippines and partnerships that encompass much of the rest of the region all mean that the United States has an enormously important interest in tranquility throughout the Asia-Pacific region. In addition, much of the world’s total commerce transits narrow maritime straights in Asia, especially the Strait of Malacca, the closure by a hostile power of which would send the world into a major economic and political crisis. In addition, the United States maintains close relations with the nation of Taiwan, which mainland China claims as a part of China—a claim native Taiwanese reject. The issue of Taiwan’s status, in fact, is one of the most sensitive in the world, as the government of the People’s Republic of China has repeatedly made very clear that a move by Taiwan to assert its independence from the mainland would be met with an overwhelming military response.
From China’s perspective, the United States is a largely unwelcome interloper within its (China’s) sphere of influence. While acknowledging that the United States is a Pacific nation, China rejects the notion of a legitimate U.S. military presence that it perceives could interfere with its affairs. The U.S. relationship with Taiwan, as noted, is a major and potentially catastrophic point of contention between China and the United States. The day may come when the United States must make a decision about whether its commitment to Taiwan’s status as a quasi-independent nation is worth the cost of a major war with China. In addition, China claims almost the entirety of the South China Sea as its sovereign territory, in defiance of the territorial claims of Indonesia, Vietnam, Thailand, Brunei, Malaysia, and the Philippines. China has a contentious relationship with Japan, not only extending from the latter’s brutal occupation of Manchuria during the 1930s and early 1940s, but over the status of a disputed chain of islands (the Senkaku to Japan, the Diaoyu to China)—a conflict mirroring that over the Spratly Island chain in the South China Sea. Chinese military activities in the region have become more overtly threatening to neighbors and to the United States, with Beijing constructing military installations on disputed islands, in order to cement its hold on them and to enable China to project naval and air power throughout the region.
So, China and the United States are rivals in the most dangerous sense of the world. Despite being major trade partners, and despite China’s role in sustaining America’s astronomical budget deficits through its purchase of U.S. Government securities, the two nations are definitely rivals—geopolitical rivals, the respective interests of which will almost certainly collide in the not-too-distant future.
Is the U.S. and China rivalry good, in any sense of the word? It is difficult to argue that a rivalry involving two nuclear-armed powers, with so many sensitive issues dividing them, can be beneficial. Mutual economic interests in a peaceful relationship certainly helps, but we are dealing with existential questions for both nations. The Chinese desperately want to emerge as a major global power with control over the Asia-Pacific region; the United States recognizes that it cannot stop China’s rise as a military power, but also knows that it cannot cede the region to a potentially hostile China, not with so much at stake. From the U.S. perspective, the rivalry is a fact of life, and one that will inevitably call into question the viability of American security guarantees for nations such as Japan and the Philippines.
The U.S. and China rivalry is a good thing from the perspective of some smaller Asian nations, who seek to play the two major powers off against each other. Myanmar (formerly Burma) has an abysmal human rights record and was the target of U.S. sanctions for many years. The government of Myanmar, however, knows that, by offering China concessions, such as military basing rights, it can leverage the United States to a certain degree. The Vietnamese are eternally wary of all major powers but use the United States as a hedge against a rising China. The government of President Rodrigo Duterte of the Philippines, which has been both a U.S. colony and an independent nation with very close ties to the United States, has moved his country closer to China and away from the United States, the former being less prone than the latter to allow issues such as human rights to influence bilateral relations. The autocratic leader of Cambodia, Hun Sen, similarly exploits tensions between the two larger powers for his own benefit. In short, to these smaller independent nations, the rivalry between the United States and China poses a threat to regional stability but also allows for a more complicated series of relationships.
A rivalry can be a good thing when it involves athletic competition. When it involves nuclear-armed submarines stalking each other and fighter jets tangling in the skies over disputed land, it is bad.
https://asiamattersforamerica.org/asia/data

https://www.theguardian.com/world/2018/feb/06/photos-beijings-militarisation-south-china-sea-philippines

https://www.weforum.org/agenda/2014/05/world-most-important-trade-route/