The arrogant attitude of William I (first king of the United Kingdom of the Netherlands) towards catholics. The south of the kingdom, Flanders and Wallonia, were primarily catholic and this caused a lot of resentment. In addition William also actively promoted the Dutch language, wich wasn't a problem for the Dutch speaking Flemish but wich angered the francophonic middle class.

Though there were other causes, I'd say Williams policies were the last drop and made the Flemish and Wallonians rebel, and proclaim their independece as Belgium. I can't blame them, but it is a pity that it had to come to this.