C is for Coffee
Coffee: a beverage made from the roasted, ground seeds of certain coffee trees.
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John didn't drink coffee, because it made him unreasonable hyper. And a hyper John would be something that no one would know how to deal with. John couldn't let himself get hyper, because in his eyes, it was the same as getting drunk. Unreasonable and possibly prone to something like a slip-up.
John was careful about such instances. He didn't need them to happen, so he made himself avoid them if it was possible. Because the last thing he needed was to slip something up when he had a loose tongue and mind.
And he didn't need that to happen.