Why do feel nervous when we are face to face with an immigration officer?

I am an experienced traveler and so I have met many immigration officers. The most friendly one was at Sydney airport. "Welcome to Australia" is far from the usual things such people say! I am asked the same thing over an over again when I enter the States. "Are you planning to work here?" I reply "no" every time. The last time I was the there they chatted for a while about the purpose of my visit and where I would be staying. The immigration officer of Guatemala was charging every person on the bus a different amount to leave the country. The poorer looking traveler's paid a couple of dollars but the rich looking older American guy was stung for twenty dollars. When I arrived at the airport in Israel I couldn't get near the check in desk. An immigration officer blocked me. I was nervous and he asked awful questions. I was forgetful because I felt under pressure. He got me taken away for a detailed luggage search! I am just an innocent traveler. Immigration officers in my home country just glance at my passport when I return home but some of my sister's pen pals have had a tough time getting into my home country. I guess I just can't help feeling nervous when I see a mean looking immigration officer.