Why did Japan invade SE Asia?

First and foremost Japan did not have its own supply of raw materials!
The rich oil fields of Dutch East Indies would enable Japan to be almost self-sufficient.
Japan invaded French Indo China October 1941 followed by Pearl Harbour Dec 1941.
The Government of Thailand provided token resistance and then allowed Japan access.
What then followed was the Fall of Singapore 15 Feb 1942 and the Dutch East Indies in Apr 1942 – bringing about access to oil!
There was now a further problem – transporting oil to Japan and elsewhere.  Japan had an increasing shortage of oil tankers.
When did the idea of Burma-Thai Railway evolve?
Probably following the invasion of French Indo-China.
By mid May 1942 Japan had successfully occupied Burma and had it’s sights on India.  Wet weather brought about a four-five month halt  – thus allowing Britain to re-organise and re-quip India  and giving force to attack Japanese shipping to Burma  – the existing route was  through the China sea, around the Malay Peninsula and up the Malacca Strait to Rangoon.
Making Bangkok the main shipping terminus would save more than a 1,000 miles and reduce potential loss of shipping from Allied attacks.
A rail link was the answer!
There was an existing railway system from Singapore to Bangkok and from Bangkok to Non Pladuk.    In Burma there was a rail link between Rangoon and Ye.
Japan had thousands of POWs to use as a labour force.