Astounding lack of common sense. Whatever personal justifications smokers might come up to explain their habits, the truth of the matter is that smoking is an incredibly stupid thing to do - it makes you smell like ass, it's bloody expensive, and oh yeah, you can (and probably will) get cancer. These are not secrets that have come to light long after the fact either, we've known that smoking is bad for you for bloody decades - if you're my age and a smoker, you absolutely cannot claim ignorance, you knew full well exactly how bad for you smoking was before you started.
So you've intentionally picked up a habit that comes with a hefty load of health detriments at best, or cancer/death at worst, knowing that you would be subject to those health detriments and/or cancer and death as a direct result of your decision, and you made that decision anyways - congratulations! You've just demonstrated an astounding lack of common sense! This is basically the reason behind any ludicrously stupid thing people do that flies in the face of all common sense and instincts for self-preservation.
Of course, people do all kinds of insanely dangerous things, and there is always an element of risk involved with even relatively mundane activities, so it is worth pointing out that it is one thing to live dangerously and flirt with death (extreme sports type stuff, where the risk is mitigated somewhat by proper safety procedures), and another thing entirely to watch Jackass and decide to reproduce those stunts at home, ignoring the warnings that make it very clear that you should never do that for any reason because that would be STUPID. The decision to start smoking, in defiance of decades of medical research telling you "That is a terrible idea" is clearly on the "Jackass" side of the spectrum.
But hey, if you're dead set on killing yourself through your own willful stupidity, at least smoking generally takes a while. Flinging yourself into the side of a barn via home-made catapult, while far more amusing for everyone who is not the flingee, does tend to be rather more immediately fatal.