I think that for the fundamental why here we have to go back much further in time then the 1930's.
Japan was essentially awakened by Commodore Perry in 1853.
Pearl Harbor - The Rise of Japan - Imperial War Museum
Japan soon realised that to have an Empire was essential to stop itself from being part of someone elses Empire. And so Japan embarked on a monumental programme of modernisation.
This led to a huge influx of westerners and western thought. Japan turned to the Great powers of the time to equip and train its forces while at the same time retaining much of its old culture.
The Army was modelled on the Prussian style while the Navy was largley built and trained by the RN.
And so Japan did what it thought a great power should do, carve itself an Empire.
There were many twists along the road to 1941. Japan probably came to the fore during the Boxer rebellion in China where Japanese forces were the only Asians that were deemed worthy of joining the European powers.
http://
www.history.navy.mil/faqs/faq86-1.htm
This no doubt encouraged the Japanese to think of themselves as co-equals with the Imperial forces, which culminated in for what at the time was the shock defeat of Russia in the 1905 war.
Japan Russia War 1904-1905
This no doubt sowed the seed for the Japanese to confirm their manifest destiny and also no doubt to confirm their racial superiority over the other Asian populations and especially China.
Their next big opporunity no doubt came in the years leading up to WW1 when the Royal Navy was withdrawing its strength back to home waters to counter the rapidly growing Imperial German Navy in what became essentially an early superpower race for Dreadnought dominance. Japan and the UK completed a treaty which would see Japan look after much British of their Far East interests. Come 1914 the Japanese gained much at little expense from this aliance and were given most of the German possessions in the Pacific.
So in 1919 the Japanese were to all purposes at the top of the Imperial game, a country to be admired and apart from Thailand the only free country in Asia apart from a China controlled by the Imperials.
We cannot blame them for this as every country, including the US had colonies in the East and I believe they were following best practice, i.e. if you cant beat them, join them.
So in the 30's Japan no doubt thought it was following normal practice to expand into Manchuria
Japan's Economic Expansion into Manchuria and China in World War Two
I hope I havent ranted on too much but firmly believe that you need to understand the background to any event. I know I have oversimplified some things but to go in depth would require a Novel.