The way I think of motivational interviewing is that the goal is you want the patient to convince themself. So the goal is to get THEM to tell YOU the arguments for why they should quit.
The patient knows that smoking is bad for them. They have weighed out pros and cons in their mind and they're stuck at "I'm still going to smoke." Smoking HAS PROs: It feels great, reduces anxiety, they're used to it, and if they were to quit they'd go through awful withdrawals. But the patient also knows that there are cons. It's going to kill them.
If you start listing all the reasons why it's bad for them that they already know, they'll want to balance out the argument for why they still smoke. You're saying the cons, so they feel the need to justify the pros. BUT if you ask them what smoking is doing for them that's good, they'll feel the need to justify the cons: "Well it helps with my anxiety, but I know that's not a good enough reason to keep smoking." You can also have them argue your side by talking in the extreme in the opposite way: "So you'd NEVER see yourself quitting?" "Oh no of course I want to quit eventually!"
Here's a good list of questions to ask in each stage: https://www.aafp.org/afp/2018/1215/hi-res/afp20181215p719-t2.gif
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