"How and why should we do it? Would the U.S. ever really allow harm to come to Canada which is, after all, the Americans’ front porch?"

This, essentially, is the reason most Canadians don't pay much attention to defense policy. We make obligatory, if minimal, contributions in order to maintain our alliances but seldom concern ourselves with much else, except occasionally with peacekeeping - as long as no Canadians get killed. But we hardly feel threatened and we have no territorial ambitions, so putting a lot more money into defense spending is and will remain a hard sell in this country. The electorate has issues other than national defense at the top of the policy priority wish list, a situation that's not likely to change.