Belgium wouldn't allow Germany passage through their territory.
Belgium wanted to remain neutral to both sides and allowing Germans passage to France was not remaining neutral to France.
Germany invaded France because they had declared war on them but when invading Belgium to get into France, they broke the treaty of London which is why Great Britain entered the war.
France was allied with Russia which is why Germany declared war against them. Hitler wanted to rule Russia and expand its empire, since France sided with Russia to defeat the expansion, Germany called for a war against them, thus pulling France into WW2.
