It is important to remember that Canada has many natural resources that Europe did not. For example, there were many beavers in Canada. Beaver hats were very popular in Europe during the 19th century, so Europeans were very interested in trading with Canada for beaver fur. Canada was also still a European colony at this time. This meant that Europe was essentially the only place that Canada could officially trade with. There were many factories in Europe and there were not any in Canada. This meant that people living in Canada needed to trade for finished materials, such as clothing as food. Canada traded their natural resources to Europe and received finished products in return. 

Why did Canada trade with Europe in the eighteenth century?

The answer to this question depends on what one means by Canada, since there was no Canadian nation at this point. If one means the Europeans who set up colonies in Canada, first the French, and then, after the Seven Years War, the British, then the answer is that Canadians traded with Europe because they were colonies of European nation-states. They were set up largely for the purpose of trade, particularly the fur trade. On the other hand, if one means the numerous Native American peoples who lived in what is now Canada, the answer is that they traded with Europeans (different than Europe) because they stood to benefit economically, militarily, and politically through trade, which was almost synonymous with diplomacy in their culture.