It has been said that we often destroy that which we love the most. While this may or may not always be the case in life as a general rule, it is most certainly the case as it regards smoking around the people that we care about. Whether or not you realize this, the leading cause of death (through things such as lung cancer and COPD) in an adult human being is through the passive inhalation of smoke, which may be nothing more than having someone near you smoking whom you do not even know… or it could also be someone that you care about doing so.
After all, we are more likely going to be hanging out with people whom we love in this world, right? It only makes sense that you would be around someone that you care about, whether or not you actually smoke, while they are smoking. And since they are in the same air space as you are, it also makes perfect sense that you are also breathing in the very same smoky, disgusting air that they are. So it should come as no surprise that not too long after they begin to develop the smoke problems that they do, you will also develop them as a consequence of your relationship with them.
Being close to a person who knows that they are doing an activity which is harmful to them is something that we do irrationally, because we care about the person themselves. And while this activity is a disgusting, terrible activity which is going to lead to their premature death from being unable to properly breathe, the love for the person who smokes can also be a habit.