Smoking proves that you officially Don't Give a Shit about your long-term health, and Not Giving a Shit is the essence of being cool. I mean, I guess for a long time now "cool" has just meant "good," but the original "cool" aesthetic was all about acting like you were probably going to die young and looking sexy doing it.
Edit: And smoking definitely contributes to the sexy part. Smoldering embers. Oral fixation. Taking a long drag and letting it out slowly. Maybe it's not for you, but it sure pushes my buttons, and I know it's not just me.
I once did some office work for a tobacco company, where you were allowed to smoke at your desk. I don't smoke, but I had a few ciggies when I was there, because you realise that you've never really pointed at anything, until you've pointed at it with a cig in your hand. It's just a better way to point at stuff (I don't make the rules).