People who have lived in both US/CA, what's the biggest difference between the two?

"Got two. I'm from Canada and I notice people hold the door for you here more often than just go straight through. The other is that when I went on a school trip to Costa Rica, I fell behind my group in Texas airport when I went to the bathroom (they were still in sight). I was headed to my group when a guard stopped me "where do you think you're going?" In a very accusatory tone. I was 17, bad matched the other kids in sight. I said I was trying to catch up to my group. "Yeah you fall behind or something?" Um, yeah, I went to the bathroom. "Mhmmmm." He was so suspicious and I didn't do anything wrong. Meanwhile on the way home in Canada it went "where are you coming from? Oh, that sounds fun! I hope you had a great time. Welcome home!" Big smile, stamps for everyone, done. "