Just Wondering... Why Do People Smoke?

It's just something I can't understand for the life of me. I'm 17, pretty athletic, and I care about my body. I'm going to bootcamp this August (USMC ALL DAY xD), and I'm pretty healthy and thank God for that. However, I have more friends that smoke than don't. Why in the world would you want to create a problem in your body when you were blessed enough to have the option to avoid it? It smells terrible, it's long-term affects are anything but good. I hear the, why do you care what I put into my body? And the I'm going to die anyways. And if I get cancer that's my problem. NOOOOOO, do you have any idea of how agonizing a cancerous death is? My grandfather served in Vietnam, and died 10 years ago of colon cancer. The cause was said most likely from Agent Orange. He didn't smoke, and I remember when he was dying, you could literally see him slowly dying, getting sicker and sicker. They had to get him on a helicopter once to get him to a hospital and he was screaming. Cancer isn't just an 'o'well' kind of disease, it's no joke. If nothing good comes from it besides a temporary relaxation, why would you do it? Besides it's super expensive, think about all the money you could've saved or better ways you could've spent it. I don't know, I just want to see a reason that actual looks somewhat legit...