Why do the major rivers in Southeast Asia run from north to south?

Most major rivers in Southeast Asia seem to flow in the same direction. Unless the land it totally flat, rivers of water run downhill. The vast percentage of rivers on the planet flow in a southerly direction because the source (usually in the mountains) is to the north of the mouth.

If the source of a river is at a higher elevation than the mouth, that river will run from the source to the mouth. However, if that (higher) source is to the south of the mouth, that river will then flow to the north (downhill).