Sure, we've all gone to the grocery store and the clerk didn't even lift his or her head during the entire transaction. But does it mean you can't say “hello, thank you and have a great day?" Any of you who have joined me via phone at the store know that when I get to the checkout I ask if you prefer to hold or if I can call you back. That gesture alone often elicits a positive response from a cashier. Greeting both the bagger and clerk by name, treating them as people not servants, grows smiles and sometimes (sadly) a look of shock or confusion. Your simple greeting may change that person's attitude towards the day. They may have just had a poor interaction with a boss, been ignored by the cell phone using customers for an hour, and so on. You making that effort to interact with the cashier... and the bagger too...and by name as a bonus...reconnects them to the people that surround them.
How many times has a friend asked you to do something “out of the box” for you and you declined because it’s just not your thing? Maybe inside you think it could be your thing but you feel uncomfortable or just assumed that because you’d never done it, you wouldn’t like, or didn’t believe you could even do it. Maybe it involves going to the beach, meeting new people, or a popular group activity that many people are doing nowadays. I mean what if you look stupid? What if no one likes you?