I wonder how much of it comes down to inconvenience. I've been bringing my own thermos for a year or so now and it took me some time to get used to the ritual of scrubbing it with soap and a bottle brush after use every day - at first I found it inconvenient enough that it made me want to just get paper cups.

Now it's a nice little ritual and I am used to the advantages of bringing my own insulated container with a lid - I can carry it back with groceries or do other errands without worrying about my drink spilling or getting cold.

FWIW I do occasionally see other people at my local coffee shop show up with their own mugs, but I agree that it's quite rare.

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