Our government takes such a hard line on border security because yours does. As a US citizen you don't need a passport to enter Canada, but because of the Western Hemisphere Travel Initiative implemented by the US governement in 2007, a US citizen needs a passport to re-enter the States. So essentially, we're not going to let you enter Canada if you can't get back into the States.The truly curious part of this equation is Canada. To my understanding most Canadians are not nearly as fearful of potential security threats as average US citizens are. From what I've read most Canadians are generally pretty relaxed about security concerns. So why is Canada's government taking such a hard line on border security? Maybe it's part of the current government's attempt to solidify their ideological bona fides. Maybe it's because they want to win points with the US government and help promote our cooperation on other controversial projects (such as Keystone XL) by following our lead or even expanding upon it. Maybe a little of both?
As to why Canadians aren't as fearful of potential security threats as the average US citizen - we have no reason to be. It may be boring and cold up here but I don't lie awake at night fearing retaliation from some country we've recently invaded.