A similar chronic-condition viewpoint to that of batsgirl: Because, you're 23 years old, and you've had a chronic condition all your life so you don't know any different; to you it's just normal life. And whilst other people might see trips to hospital as something rare and scary, you are completely used to going into hospital and you feel comfortable and feel safe there, and your trips there are annoying because they are a frequent disruption to your routine, nothing to get worked up about. And you discharge yourself from the resuscitation room against medical advice because you'll have done so countless times before, without having any problems. You get fed up with being told to slow down, take it easy, take precautions. You have a live to live; you may be missing work, or a dinner with friends, or a carefully-planned trip to IKEA - there's stuff to do - you don't need the hassle! You are certain that you know your own body better than the doctors and that you will be OK if you leave rather than hanging around wasting time in a hospital. Unfortunately this time you were wrong. And you smoke because you're just like any other 23 year old that smokes: you think you're invincible. Also, you don't want to admit to yourself that you are in any significant way incapacitated or different to anyone else. That is why.