There is no good reason.

A fence of this length is a waste of resources and ineffective as the Chinese and the French found out.

I get BDJ's joke.
I even laughed out loud.

I doubt that fences on either side are the answers! Enforcement of the laws and close scrutiny of those allowed to visit here seems a much better plan along with our military patrolling our borders and doing the job of our federal government, Protecting our borders! Given a choice I am sure our soldiers would much rather be in the deserts of TX and NM and AZ than in Iraq, Afghanistan or any other Middle East country.

In the age of terrorism, building a fence on both borders is the order of the day, especially in an age of political correctness, where one can't call a spade a spade.

We've already had one terrorist intercepted crossing the border with Canada, who was intending to bomb LAX, and the Times Square bomber came from Canada. Navigable roads are harder to find on the Northern border, but given a few more incursions by terrorists, and expect to hear a call for more security there as well.

Because that's the source of the illegals.
Duh!

If things keep going downhill in the U.S., the Canadians may want to build one.

because Canadians already are on the dole :)