Why are employees of the United States' customs so rude?

I travel from the United States to Canada and back all the time, and I have never ONCE met a respectful U.S. customs officer. They are always so angry looking and serious, and some seem to want to put a bullet in your head for absolutely no reason. I understand National Security is of utmost importance, but I feel they can still do a good job protecting our border while being respectful or at least have a smile on once in a while...any thoughts?