China is "communist" in name only, but they are still a one-party totalitarian state, which is a big reason liberal democracies and "communist" countries weren't friends in the first place.
In fact, if "the state/people own the means of production" had been the only aspect of communism (and not the totalitarian dictatorial governments), I doubt the Cold War would've been as serious.
Some of it is the unresolved territorial dispute over Taiwan.
Some of it is opposing economic competition.
Some of it is bi-directional; there used to be a lot more rapprochement until relatively recently, but changes in internal CCP politics have led to China using an external enemy to bolster internal unity. An exact mirror of the US in this regard.
Hopefully it will remain no hotter than the odd "shoving war" that Chinese and Indian border guards engage in, at a remote border post where neither side is allowed guns.