From circa 1925, Japanese political/social culture shifted significantly — away from the democratic experiments of the Taishō era and towards the nationalist, militarist, expansionist and ultimately totalitarian state structure that characterised the Shōwa period.

So, for example: the Peace Preservation Law of 1925 massively reduced personal freedoms and squashed political dissent, and under a series of prime ministers Japanese foreign policy evolves in direct opposition to the Western powers (widely seen as having manipulated, strong-armed and Japan since the end of Sakoku.)

By the late 1930s, they had developed the concept of the Greater East Asia Co-Prosperity Sphere; the idea of a Japanese empire in east Asia as a national right. The idea was that Asia shouldn't be part of a European sphere of influence; they took "Asia for the Asiatics!" as their slogan — and believed that, naturally, Japan should and would be the dominant influence in the region.

The Japanese regime from the late 1920s until 1945 is commonly described as 'fascist', but that's not strictly true. Japanese politics in the post-Meiji era is a case study in syncretism; it simply doesn't fit the conventional Western left/right paradigm. Japanese imperialism is a similar case: it doesn't fit the colonial model of the Western powers, but was undoubtedly influenced by it — it's been described as 'mimetic imperialism'.

But fascist or not, Japan identified ideologically more closely with Nazi Germany and Fascist Italy because those regimes also saw themselves as natural opponents of and successors to the traditional, liberal-democratic global powers (Britain, France and the United States.) You see that goal explicitly stated in the text of the Tripartite Pact (the 1940 military alliance between Japan, Germany and Italy):

The governments of Germany, Italy and Japan, considering it as a condition precedent of any lasting peace that all nations of the world be given each its own proper place, have decided to stand by and co-operate with one another in regard to their efforts in greater East Asia and regions of Europe respectively wherein it is their prime purpose to establish and maintain a new order of things calculated to promote the mutual prosperity and welfare of the peoples concerned.

[...]

Accordingly, the governments of Germany, Italy and Japan have agreed as follows:

ARTICLE ONE

Japan recognizes and respects the leadership of Germany and Italy in establishment of a new order in Europe.

ARTICLE TWO

Germany and Italy recognize and respect the leadership of Japan in the establishment of a new order in greater East Asia.

In the late 1930s, Japan saw opportunities for expansion in east Asia: they invaded China in 1937 (the Second Sino-Japanese War) — that, incidentally, is in large part why China eventually became one of the Allied powers. Their invasions of Western territories in Asia — Hong Kong, Singapore, the Philippines, French Indochina, and so on — had dual motives: to secure strategic resources (oil and rubber, in particular) and to break Western colonial power in the Far East.

Japan came into conflict with the US ultimately as a result of a US-led oil embargo that cut off almost 90% of Japan's oil imports, leaving them with just three years of petroleum reserves. The attack on Pearl Harbor was in large part an effort to knock out America's strategic influence in the Pacific.

A couple of good overview from the historiography on Shōwa-era Japanese foreign policy and imperialism:

• W. G. Beasley, Japanese Imperialism, 1894-1945 (1987)
• R. Myers & M. Pettie, eds., The Japanese Colonial Empire, 1895-1945 (1984)

There's plenty more that's worth reading — the historiography on Japan is huge. But those two are a good starting point.

Japan and Germany had been aligned since the 1936 Anti-Comintern pact against communism in general and the Soviet Union in particular. Around this time, Japanese militarism was at an all time high. Their military wanted to expand Japan's borders to gather the resources needed to fuel her empire. Hence their outright seizure of Manchuria during the Mukden incident of 1933. This in turn led to severe Sino-Japanese tensions that had several outbursts of violence shortly before the Marco Polo incident. The Mukden incident itself left a lasting impression on the military: namely, that it could essentially do what it wanted and the civilian government could do nothing about it. We saw this during the Marco Polo bridge incident, where ironically the man who was virtually single-handedly responsible for the Mukden incident, now promoted to general, tried to stabilize the situation but was essentially ignored by the more militant factions of the military. The Second Sino-Japanese War started, and it antagonized many of the Western Powers.

However, in 1939, Germany and the Soviet Union signed the Molotov-Ribbentrop Pact. This infuriated Japan as they believed that Germany had just stabbed them in the back. It also didn't help that Germany had been on very good terms with China until 1938, including sending military advisers, industrial support, arms dealing, trade, training cadres, and so forth to support China's effort to modernize and fight the Japanese (which is in itself a fascinating topic but for another time). To try and soothe the Japanese, the Germans and Italians formalized the new Tripartite Pact to show that they were not abandoning Japan, whom Hitler wanted as an ally against the Soviets. Unfortunately for him, the Japanese ended up signing a non-aggression pact with the Soviets and didn't want to denounce it as it would expose Manchuria to a Soviet attack. So when Germany invaded in June of 1941, Japan decided to maintain the pact. Diplomacy is hard, yo.

The problem with Japan feeling like it needed to take sides was that it had already felt that the West had betrayed them after WWI. They were denied a large amount of the territory that they had wanted, and they were denied a "racial equality" clause in the Charter of the League of Nations, which was intended by Japan to give Japan equal recognition with the other Powers but scared the colonial powers. There was also considerable angst from the Washington Naval Treaties, where Japan, already smarting from Britain's abrogation of the Anglo-Japanese alliance, was given a smaller fleet allocation than the US and the UK.

The real kick in the nuts, so to speak, was the US oil and especially scrap metal embargo in 1940 in response to the Sino-Japanese War and the Japanese occupation of French Indochina. While as k1990 says the Japanese were running out of oil, they were running out of scrap metal even quicker. To secure these resources quickly, they chose to invade the Dutch East Indies. The problem with invading the DEI was that Britain had a firm commitment to defend Dutch sovereignty, and that the Philippines was between Japan and the Dutch East Indies, being an existential threat to a hypothetical supply route between the DEI and Japan. Thus, for Japan to secure the DEI, the only feasible option was to attack the Philippines and by extension the United States.

Then again, the only "really" feasible option for Japan by the time the embargo came around was to stop the war and beg for mercy, really.