was somewhat neutral in WW1 and in WW2

In both cases they were firm members of the allies. The Belgian government in exile and the Belgian Army in exile both contributed to the allied cause in WW2.

Remember, Belgium didn't become a country until 1831.

Belgium is perpetually neutral (unless attacked), because it was one of the conditions of the Treaty of London from 1839, which ratified Belgian independence from the Kingdom of the Netherlands.

After the final defeat of Napoleon at Waterloo, the great powers decided on the need for a middle power state bordering France. Since the former Republic of the Seven United Provinces (a.k.a. the Netherlands) had shown to be an inefficient deterrent to French aggression, the formerly Spanish/Austrian Netherlands (a.k.a. modern-day Belgium) were united with the Northern Netherlands in a new Kingdom.

The Kingdom itself was a failure, and only 15 years later (in 1830) Belgium broke off. It was de facto independent for nine years, before the Dutch king William I acknowledged defeat. At the Treaty of London, 1839, Belgium independence was ratified (and modern day borders between Belgium and the Netherlands established). One condition was the perpetual neutrality of Belgium.

Another article was the guarantee of independence by all the great powers. It was this article that "forced" the United Kingdom to declare war on Imperial Germany in 1914.