I recently made the comment that quitting smoking is as simple as not smoking the next one. What could be easier than that, right? I know that quitting smoking isn't really "easy", and I do often think about why it is so difficult to quit.
I've come to the conclusion that the difficulty lies in the fact that we focus on the reasons why we shouldn't be smoking and yet want to smoke, rather than focusing on the reasons why we should be smoking and the justification for why it would be good for us.
Try building a list with "Why I should NOT be smoking, with all its detriments" on the left hand side of a page, and "Why I should be smoking, with all its benefits" down the right-hand side.
I think you will find that trying to justify the benefits of smoking might leave you somewhat short of ideas