The first is easy to explain. The US likes being number 1, and if anyone else, whether the Chinese or the Japanese or the Indians looks like they're going to be number 1, the US doesn't like it.
The second is this: for the last 40-50 years, the US and China have been in a hugely mutually beneficial relationship, at least on the corporate level. American companies outsourced many of its lower wage lower skilled manufacturing to China and China has saved those companies a ton of money and made their owners a lot more profit. In order for China's economy to keep growing though, the Chinese believed, correctly, that they needed to move up the food chain since their labor is not as cheap as it once was. So they identified 10 "industries of the future" where they want their companies to dominate. American companies are, once again correctly, worried that since these are the industries that we would also like to dominate, that the Chinese, if they are successful, will literally eat our lunch.
There's a bunch of other stuff relating to the South China Sea, Taiwan, forced IP transfer, hacking, trade subsidies, etc. that are also causing tensions, but those two are probably the biggest ones.