Question

why was Brussels chosen as the headquarters of the European union

Answer 1

The Brussels was chosen as the headquarter of the European Union as the Brussels became as a role model for managing diverse ethnic population without any of the conflicts. So as a part of this it also represents many of the advantages:
1) Brussels is centrally located
2) It has a socially diverse population
3) It is a cosmopolitan city.
4)It is situated in the center of the Urban Europe  and has a good connection between the major cities like London, Paris etc.

Answer 2

Brussels is a small country in Europe surrounded by traditional continental powers of France, Italy and Germany.
When the idea of developing the European Union came about, there was a need for trust, neutrality and stability between all countries who were previously bitter enemies with one another.
In order to provide a powerful, save and neutral ground to the development of the European Project, Brussels was chosen was the Headquarters of the EU, representing no major European power, but a multi-lingual and diverse population, similar to Europe as a whole