There is nothing wrong about secession, per se. In fact, in many cases it is the best solution when two or more different peoples living in the same country have very divergent goals, and especially when one is the victim of repression. But of course, as Captain Amazing mentions, the country that is internationally recognized will almost always oppose secession, since it causes them to lose territory and resources. (The only exception I’m aware of is Singapore, which became independent as the result of being expelled from Malaysia.) As well, it is frequent that one or more of the new entities will suffer economically as a result of secession, but that doesn’t necessarily make it a reason not to attempt it.
Some people in this thread have suggested that if we allow secession, we get on a slippery slope that will allow any group, up to the individual, to secede. I disagree. In almost every case, the entity that wants to secede from an independent country forms a sociological nation. (Now, I’m sure there are cases where, say, an American municipality wants to secede from its country to join the neighbouring one, but I won’t consider this as it is solely an interior matter.) A nation, basically, is a group of people sharing a culture and a sense of belonging together. They may or may not be defined in terms of ethnic group of common language. I must admit that nations are a rather “fuzzy” concept, in the sense that it’s difficult to understand why some group forms a nation and some other doesn’t. But when they exist, they are recognizable. I’ve heard them described as “imagined communities”, but what is clear is that here, imagined emphatically does not mean imaginary.
In the current case of Belgium, to discuss fetus’s example, it is quite clear that Flems and Walloons are two different nations. They speak different languages and have a different view of their history, and don’t necessarily have much contact with each other. Now, this doesn’t necessarily mean that they should form two different countries. But they appear to have different views about the future of their country. Their political disagreements (Flems tend to favour liberals while Walloons tend to favour socialists) are making the country very difficult to govern. As well, Flems are tired of the perceived arrogance of the Walloons towards them. Most Flems speak Dutch (Flemish) as well as French, English and possibly other languages, while Walloons rarely speak Dutch. So they feel that they are stuck in a country with a neighbour that doesn’t care about them, and that prevents them from reaching their potential. This is why many of them are starting to think about secession. (Unlike fetus, I’ve never heard of secessionist Walloon groups.)
Now, secession of Flanders from Belgium isn’t necessarily the best idea. In fact, in this case, it seems to me that a more decentralized federation might be a better idea. But I don’t get to decide, this is something Flems and Walloons will have to decide. But I want to respond to Whack-a-Mole’s comment about secession being the thing of “a few craving power”. This is emphatically not the case. In the secessionist movements I know about, the members hold the ideas they do because they think that membership in their country hinders their nation’s progress. The power they get to wield doesn’t have anything to do about it (at least, not more than any other politically active person or politician).