WHY did Germany choose to go the western side of Europe first?

The diplomatic and military context of Europe in 1940 and its circumstances may explain why Hitler decided to attack France.

After the invasion of Poland in September 1939, France and Great Britain had declared war on Germany, but it wasn't accompanied by military operations. It was the so-called "phony war", which together with bad weather, undoubtedly gave Hitler and his commanders enough time to plan the amazing Blitzkrieg with which they overwhelmed Belgium and France.

Before invading Poland, Hitler made a non-aggression pact with the Soviet Union; he assured his back and neutralized the worrying possibility of fighting a two-front war at that moment. Germany was not prepared to attack the Soviet Union. So, he turned all his attention to try to knock down his main threat: France and Britain. Belgium and Holland were neutral nations and were taken for surprise.