i think they reason they are stern with everyone and not happy go lucky is that people who have something to hide are probably more likely to react in a different way than people who have nothing to hide when they are placed in a stressful scenario rather than dealing with someone who addresses you like your friend. that is the reason why they ask a lot of random questions to see if they can trip you up. if they think you're responses and body language is not normal they may warrant more searching or questioning.
i've had some interesting experiences at the border. going to florida my friends and i didn't have anywhere booked to stay the first night because we were driving in one go and we didn't know when we would get tired and potentially need to stop. our entire car got unpacked and searched while we sat in a small room with a camera mounted in one corner focused on us. we got questioned let go and all was good. there were other times that my car or my friends cars got searched but since i have nothing to hide i don't mind this terribly. it makes for a good story later.
anyway, i think canada and the us should have a schengen-like agreement. it would make more sense, but then canada wouldn't make money off of duty.