I've been through immigration (Washington State <-> Canada) multiple times (like more than 50 times) and I have NEVER had any issues with American immigration/border patrol. I was born in South-America (brown complexion and with an accent), but am an american citizen. Even when I was a permanent resident never had issues with them. Sure, I was detained various times (only a few times for more than 1/2 hour), but didn't feel it was an excessive amount of time. Early on, I learned that if you keep your cool, you amplify your chances of not getting flagged.

Now, about Canadian border patron...

Even though they might ask you "What brings you to Canada?", the way they really ask it, while they're condescendingly looking at you is "WTF are YOU doing in my country?"

I have been harassed so many times by them. All with no smiles, no niceness, nothing. I stumbled on an oddly worded question once... this flagged me right away. They made me feel like I was lying, and even had the audacity to tell me: "If you're lying to us; NO CANADA FOR YOU!".

IMO, American border patrol wants to get you on your way as soon as possible after they do their own checks (whatever those are). Canadian border patrol want to harass, intimidate and annoy you to the point where you just want to say "F* YOU, I'm going back to my country!"