By deciding they give zero fuks about their health. Usually by deluding themselves as to how much damage they are causing by minimizing it.
Some people have an insane health tolerance to it, though. I had a grandad who drank like a fish and smoked like a chimney, and the guy lives into his 80's. I don't think I'd make it very far into my 40's if I had done that as long as that man did.
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