Every now and then on a long drive I'll somehow forget to wave thanks if I'm let through. I IMMEDIATELY feel AWFUL.
Yet, at work, I'll hold the door open for some people and they LOOK at me and don't say a word. I don't understand how it's even possible to not acknowledge someone who does that. I'd MELT with shame.
I can relate to this. My car has tinted windows (not in a GANSTA way) just come as standard - especially the rear ones. I find myself waving acknowledgment sometimes, and worrying "Will he have seen that? I'd hate to look ungrateful!"