A general trend I have noticed in French history, dating from the fall of France after the defeat at Waterloo and the subsequent losses in the Congress of Vienna, is that there is a strong urge to regain a sense of international prestige. Through out the nineteenth century France slowly degrades from being the world capital of liberal democracy and intellectualism. In the latter half of that century, France suffers an embarrassing and crushing defeat by the newly formed power of Germany in the Franco-Prussian war. This defeat is then compounded by the utter devastation of what was once the most feared military force in the world.
Even in much of the 20th century France is seen as a major world power. As a personal aside, my mother's generation in the Netherlands learned French as their first foreign language, and despite the rise of America, it wasn't until roughly generation X that English replaced French in this regard. Taken all together this explains much of France's attempts to maintain their cultural identity and obtain positions of international importance (such as their attempt to be the home of the EU government among other things). While there are some economic arguments that can be made on the behalf of French colonialism to justify French action in Indo-China, they seem to be much less important than the irrational cultural motivation for doing so (similar to America's need to invade overcome the "Vietnam syndrome" to 'justify' the overblown invasion of Grenada).