Why is Southeast Asia so densely populated?

by augustbeard

I'm curious what the historical reasons for the high population of China, India, Indonesia, Pakistan, etc. Especially after seeing this map: http://i.imgur.com/CK6aONG.jpg.

KexanR

Just a small note: By convention, South East Asia usually refers to the collection of smaller states that are east of India and south of China. East Asia would be China, Korea and Japan and South Asia is usually used to refer to India and the region around it.