US border guards are primarily ex-military. Canadian border guards are often civilians (in fact, a good friend of mind worked at CBSA during summers between semesters at university). This lends a lot to the attitudes of the guards.
That said, I've always had an easy time going through the border, and have rarely had more than a couple questions asked to me over the many many crossings (this may be because I have a student visa in my passport and I believe that answers most questions for the guards before they ask them). One tip in contrast from a post above -- Don't think too much about what you're going to say, because if they ask something else you won't be prepared. Imagine border guards are actually people and you're just having a conversation with them!
Ex-military, like everyone else in the world, are individuals. Being in the military doesn't turn you into an asshole, although it could amplify that tendency if you went in with it. Military and ex-military are not exotic aliens, they're us, nothing more and nothing less.
I think it's more likely that poor treatment at the border is institutional.
The Stanford Prison Experiment seems to contradict your claim that normal people can't/won't be conditioned to develop hostility in "us vs them" scenarios.
I don't claim that normal people can't or won't be conditioned to develop hostility in us vs them scenarios.
I also don't claim that being in the military doesn't change people. Everything changes people, and the change in the military can be profound; in my opinion that change will most often be positive. I am biased, being ex-Navy.
What I'm saying is that the military doesn't produce assholes to the extent that you can reliably explain that behavior as a result of having been in the military. They're individuals.
I do not agree that a military, ex-military, border agent, or other law-enforcement agent's inherent personality can be separated from his personality "on the job".
All sorts of executive-branch military or law enforcement posts seem to encourage "us vs them" thinking, while shifting the bright line in the "us" direction. Accepted norms become narrower, while "the other" becomes correspondingly broader.
That change is not confined to on-the-job attitudes and behaviors.
That said, I've always had an easy time going through the border, and have rarely had more than a couple questions asked to me over the many many crossings (this may be because I have a student visa in my passport and I believe that answers most questions for the guards before they ask them). One tip in contrast from a post above -- Don't think too much about what you're going to say, because if they ask something else you won't be prepared. Imagine border guards are actually people and you're just having a conversation with them!