Interestingly enough, I had the exact opposite experience as a Canadian camping in the US this summer. The customs at the US was fantastic, it was our first time visiting, and my 11 year old daughter was very nervous. The customs office asked questions in a nice way, smiled at my daughter and told her not to be so nervous, indicated that we'd have a great time where we were going, and generally was very friendly and welcoming.
Coming back into Canada, the customs agent was not nearly as friendly. Perhaps we looked a little scruffy after a week of camping, but the questions were more brusque and suspicious.