China insists that its military spending is purely for defensive purposes. What China does not comment on is the two decades of officially sanctioned media activity inside China, wherein the United States is portrayed as the opponent in a future war. The books, articles and films all make this future war sound inevitable. China does not allow, officially at least, this stuff to be translated. But Westerners can obtain it, and some has been translated. Once you go over this material, China's defense spending, and their recent protests about it, make sense. The Chinese leadership, a bunch of Communist Party politicians, are desperate to keep their police state going. Creating a credible external enemy, and mobilizing nationalistic fervor, is a classic way of retaining power.
While I don't think we must be enemies, I tend to want to err on the side of being strong and assuming the worst but acting nice. Then, perhaps the Chinese will fear us long enough to evolve into a less aggressive stance.
Or who knows? Maybe we'll become close friends.