I've always viewed a "thank you" as something to be offered when you have received something but have nothing else to offer back. For example, if someone is holding a door open for me, I will say "thank you." If someone gives me money, I will say "thank you." If someone offers me a compliment, I will say "thank you."

In the case of fast food, however, you are giving a store currency in exchange for food. Both sides are already receiving something. As such, I don't view a "thank you" as being a necessity. I'm paying for food, not for cheap niceties. Sure, it's nice to get a "thank you" from the kid working full-time handing a bag out a window for minimum wage. But if I don't get one, it's not the end of the world. I wasn't promised a "thank you." I was promised a burger, and if I get it, I'm happy.

If you're upset by these kinds of things, more power to you. But personally, I've got better things to worry about then if society is getting progressively ruder. I'm more concerned about dealing with the things I do have control over (i.e. my conduct towards others), rather than the things I don't (i.e. other people's conduct towards me).

Thank you.