Why can't people be polite?

I'd like to think that I am the kind of person who doesn't let the little things in life bother me.

For the most part, I let things go like water off of a ducks' back. Try as I might, though, there is one seemingly small upset in life that drives me absolutely crazy!

People who do not say "thank you" when you extend a courtesy to them are my pet peeve. If I hold the door for someone, especially while my arms are full, it's raining and my phone is ringing, I'd really appreciate a thank you. It doesn't take long.

I honestly believe one of the reasons we're getting more rude as a society is because of the minority of people who don't acknowledge someone's kindness.

Why should we bother being polite if it won't be returned? If I let someone go ahead of my in a grocery store line and there's no words of thanks, I immediately regret my decision. First I can feel my blood pressure start to rise, I can feel my heart rate increase, and my palms usually start sweating. It probably sounds a little wacky, but I just cannot stand it. I'm not vocal about it, though, and I sometimes envy people who have the courage to say something like "You're welcome!" or "It's only polite to say thanks."

Maybe I wouldn't find myself so stressed out and being driven so crazy if I did let it out once in awhile. Something that agitates me equally is people who don't say "Excuse me" when they push past or bump into me. Where has common courtesy gone?