What was Japan's primary incentive to attack the United States?

The Japanese had pursued conflict in China since, depending on how you look at it, either 1931 (with the conquest of Manchuria), or 1937 (with the beginning of the second Sino-Japanese war). This conflict turned out to be one of undefined objectives in which the goals consistently grew, to the point they were so poorly defined they were unreachable—today we call this mission creep, and it is important in this context because it prevented Japan from coming to any kind of settlement in China since it was never going to be possible for them to pull the 'well, we won anyway' card.

During this time, antagonism with the US had risen as a consequence of Japanese actions in China, both against the domestic Chinese and against the US. For instance: in December 1937, as the Japanese advanced along the Yangtse river, they strafed and destroyed the US gunboat USS Panay (later paying compensation.)

Before this, it should be realised that a lot of Japanese emigrated to the United States, and laws were eventually put in place preventing this. The United States argued forcefully against Japan in the Washington and London Treaties and the Japanese nationalist intelligentsia felt the agreements in those treaties were a diplomatic attack on Japan; the treaties limited the amount of warships that countries could build and Japan had less than the United States and Great Britain.

During 1940 and 1941 (in more than one stage, contrary to general knowledge), Japan increasingly took over parts of French Indochina, starting with the demand to station aircraft in Tonkin. This desire was not to do with expansionism, but because Japan wanted to control the flow of supplies to its enemy, China, through the Tonkin railway. (Source: The Decline and Fall of the Japanese Empire by John Toland). Over this period the United States enacted a number of diplomatic-economic measures, including an embargo first on scrap metal, then a freeze on Japanese assets, then an embargo on oil (as well as a number of other, consequential but not major, materials.) This was seen by the Japanese as a shadow-declaration of war.

Negotiations between the two countries continued. I won't go into full depth (mainly because I do not remember all of them; there were a lot, based around different premises) but some major factors in their collapse were:

Japanese obstinacy
Poor translation/cultural understanding between sides
(in my mind) the serious Japanophobia of Hull's chief pacific adviser, Stanley Hornbeck
A US belief (partially right) that the Japanese were not being sincere and were playing for time.

The Japanese proposed two plans; A and B, to the United States, which were two Japanese 'peace offers', both of which were obviously pro-Japan making gains in China. The United States returned with the Hull memorandum (named after Sec State Cordell Hull) which demanded Japanese withdrawal from China. The Japanese interpreted this as meaning ALL of the Asian continent, including Formosa, Manchukuo, and Korea, and decided the Americans couldn't be negotiated with and would go to war.

It was utterly impossible for the Japanese to simply 'pull out' of China. They had spent almost all their national treasure and more than 100,000 of their young men, completely mobilised their country, and told their people for four years that it was a divinely inspired war that would almost certainly result in a Japanese victory; a rightful one with an obvious moral foundation of anti-colonialism and a friendly assistance to the Chinese people. Meanwhile their aims had never been obviously stated and confirmed, so a withdrawal would always look like a retreat, even though they had won every single battle they had ever fought in China up until they point (excluding, possibly, the 100 Regiments Offensive and the Battle of Changsha in late September 1941).

With the Hull Memorandum, the United States had delivered them, in Tojo's words, 'an ultimatum'—they had no choice but to attack, since withdrawal was political suicide. I believe we can take serious note from this today, even in liberal democratic societies, about the dangers of war with undefined objectives.

As for their people, if you read accounts—I have since forgotten the name of this book but I will list it when I get home—Japanese people weren't necessarily pro-war or anti-war. It was more about duty. They had been called to fight by the Emperor and had no choice. Only later on in the conflict did the hardliners, who really believed some crazy things, seriously take over the institutions, and even then not completely. They had, however, been indoctrinated, in the main, into believing their innate superiority. I think most 'easy-answer' historical works tend to note that, but I would say that during the 20th century war was really a banality, and most people fought in major wars just because it was their duty to fight for their country if called. I doubt that the average Japanese held any more ideological basis or hyper-nationalism than the average Briton or Italian. Their institutions were just more rigid and authoritarian. That is the image I drew from the book, anyway.