Yeah. It's an automatic response. I can't even remember my mom and dad enforcing manners... they just led by example. I feel awkward when it's a double entry and the person holds both doors for me... back-to-back thank yous... awkward. But like I said, it's automatic. :P
I also wave thank-you and you're-welcome when traffic is heavy and someone makes a space for me or I leave a space for someone else. I have a "good morning" for everyone I encounter on my daily run. I ask before joining a line to make sure I'm not cutting anyone, if it's not obvious. And I definitely don't feel as if I'm in the minority. If you look for the good manners, rather than focusing on the bad ones, you'll see there are plenty of courteous, polite people out there. Unfortunately, as humans we tend to fixate on that which annoys us. ;)