From what I recall, Mein Kampf makes no suggestion of western conquests, and the Siegfried Line was only finished at great expense in the summer of 1939 to secure the Western Border and allow Eastern Expansion. With this in mind, why didn't the Wehrmacht feel the need to go on the offensive in 1940? After all, it was Britain and France that declared war on Germany, not the other way around.
Opening note: I think there's a typo in your post, as you say "why did Hitler feel the need..." in the title but "why didn't the Wehrmacht feel the need..." in the post.
The French army was at the time considered the number one threat to the Wehrmacht in Europe. Hitler may or may not have wanted to eventually settle scores with France, but while it's true that the French state was not a territorial objective of the Germans in the same way that the USSR was, the French and British had put themselves in Hitler's way with their guarantee of Polish independence. Poland was a hated country, it was also a country that would need to be conquered before the Germans could advance into their long term goal of the Russian Hinterlands.
While the French and British did not press the attack in 1939, they were clearly building up war material, making orders to American armament firms, and summoning the vast resources of their Empires. They were at war, the Wehrmacht could not just sit there and ignore them indefinitely. The Wehrmacht had a head start in rearmament, operating something resembling a wartime economy from the mid 1930's and pushing their economy as far as they could by 1938/9 (see Adam Tooze's 'The Wages of Destruction' for an excellent summary of the German economic situation, and the scale of the economic disadvantage they had prior to the Fall of France). But the initial German advantage would become diluted over time, the core territory of France and the United Kingdom had ~120 million citizens to Germany's 80, not counting colonies and dominions. Germany would not win the fight if it descended into an economic slugfest as World War I did. They ''needed'' to neutralise the French army before the British could transform itself into a first rate land power alongside France and fully bring their material superiority to bear. The only way this could be achieved would be by delivering a knockout blow in a relatively short campaign, which is as it turns out exactly what the WM achieved in 1940.
If the Germans just sat idle in the Sigfried Line for 1940, it's questionable as to whether they could have defeated the Franco-British forces the following year, let alone gone on the offensive against the USSR while the French and British were massing on their western border.