Answer:
The Europeans were interested in colonizing Southeast Asia for the same basic reasons that they wanted to colonize the other regions of the world.
1. natural resources
2. cheap labor
3. to collect taxes
But with Southeast Asia, there was a new reason for wanting to colonize-- it was a backdoor to China. The Europeans really wanted to do trade with China, but China wasn't interested. Therefore, Europe quickly colonized China's next door neighbors so that when China finally did open up its doors to trade, Europe would be right there ready to go. Vietnam, Cambodia and Laos became French colonies while Burma went to Great Britain.
Thailand was the only country in Southeast Asia that was never colonized by the Europeans. Thailand remained independent throught the colonization period in Southeast Asia. However, Japan did conquer Thailand in the early 1900's and made it part of the Japanese Empire.
Indonesia was a Dutch colony (Holland) while the Philippines was Spanish.