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.