So he just stopped. Like that.
Because he found it so easy he always hates it when he hears other people say how difficult it is to stop.
EDIT: And @Valiums - I don't want to sound sexist - but it is worse for women - and I think that, as a fellow INTP, you will appreciate my reasoning:
I heard many years ago at school during an anti-smoking lecture (and I think it is based on good science too), that it has a negative impact on a woman's ova, and thus reduces the potential IQ of any children she may have -- even after she has stopped smoking!
I value intelligence, and though if my children were less intelligent than I ideally "wanted", and though I would care for them just the same, I would rather they were given the best "opportunities" to begin with in that regard.