Why did military japan choose to conquer Asia-Pacific?

First, the Japanese had almost no natural resources. Rubber, tin, iron, et cetera are found in abundance in SE Asia, but not in Japan. Japan wanted an empire, and empires need resources. So, the thinking was, "why not take it?" And take it they did.

Japan came up with this "Greater East Asia Co Prosperity Sphere" as spin and P.R. to get the Southeast Asian nations to buy into this idea. It was pure hokey.

Second, the Japanese fought with the Allies in World War I. They expected to get Germany's pacific colonies and more territory at the Treaty of Versailles conference. The Western powers virtually ignored Japan and treated the Japanese representatives like 2nd class citizens. This did not sit well with Japan, so Imperial Japan felt it was their right and their due to go GET an empire. If you aren't going to be given some of the spoils of war after fighting on the winning side, you're going to go take it.

I say this not to defend Japanese actions during World War II. I strongly condemn Japanese atrocities during the War. A Japanese artillery shell permanently blinded my grandfather and killed many of his unit on Papua New Guinea in 1942. Nevertheless I hope this explains why the Japanese invaded southeast Asia.

How were the lives like duing militaristic Japan?

 
The Army and Navy had virtually unlimited freedom in terms of what the could do during this period. And their actions reflected the remarkable irrational and undemocratic character of the military. The Japanese military rule, led the entire country into a war that could not be won, and pushed it to the point of national destruction. During the period of the Meiji, the military was placed under the authority of the council of state, making it so that there was no separation of military and civil affairs.

Reasons that led to the rise of military japan by the 1930s

Failed democratic government

Diet could only question and had no power to make decisions and no power over ministers and nation's budget.Zaibatsus supported government with fundings so they could influence government policies.It passes laws benefitting Zaibatsus.JThe government was unable to solve economics problems.Japanese viewed the government as useless and corrupted. They looked to other alternative parties.  

 Great depression                                                                                                         Japan's prosperous economy and new government structure did not continue into the next decade. Japan, an island nation with few natural resources, relied on foreign trade. When the Great Depression hit the world in the early 1930's, counties no longer imported Japanese luxuries such as silk. The value of Japanese exports dropped by 50% between 1929 and 1931 (p 686, Beers). Many people blamed the government for Japan's economic crisis. After Japan agreed to keep its navy very small at the London Naval Conference in 1930, the military and nationalists became fed up with the government (p 686, Beers).Since Japan lacked natural resources and building space, the military invaded the area of Manchuria in northeastern China in September 1931. After the Prime Minister resigned and army officials assassinated the next, the military organized a Japanese military dictatorship (p 686, Beers). The military takeover was not a violent one, and was not led by a specific leader. The constitution and Diet remained in place, but throughout the 1930's Japan became much like a totalitarian state, but with a group of leaders instead of a single dictator. Critics of the governments were arrested, and students in school were taught to unconditionally obey the emperor and the government.
The League of Nations did not take action against Japan's invasion of China. In 1933, Japan withdrew from the League (p 687, Beers). Japan continued to invade China, easily crushing all opposition. Northern and central China was under Japanese control by 1938 (p 698, Beers). In 1940, Japan became allies with Germany and Italy, who helped Japan fight against the United States during World War II.

Economic factors

During the 19th century, Great Power status was considered dependent on resource-rich colonial empires, both as a source of raw materials for military and industrial production, and international prestige.

Due to the lack of resources in Japanese home islands, raw materials such as iron, oil, and coal largely had to be imported. The success of Japan in securing Taiwan (1895) and Korea (1910) had brought Japan primarily agricultural colonies. In terms of resources, the Japanese military looked towards Manchuria's iron and coal, Indochina's rubber, and China's vast resources.

Introduction to Japan

 Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south. The characters that make up Japan's name mean "sun-origin", which is why Japan is sometimes referred to as the "Land of the Rising Sun".

Japan is an archipelago of 6,852 islands. The four largest islands are Honshu, Hokkaido, Kyushu, andShikoku, which together comprise about ninety-seven percent of Japan's land area. Japan has the world'stenth-largest population, with over 127 million people. Honshū's Greater Tokyo Area, which includes the de facto capital city of Tokyo and several surrounding prefectures, is the largest metropolitan area in the world, with over 30 million residents.

Reference:http://en.wikipedia.org/wiki/Japan