Why do people put carcinogens in their body when I'm wanting them out?

As I sat in the clinic operatory today waiting for the shot to help my white blood cells come up..I stared out the window. I saw a girl walking toward the clinic smoking a cigarette..I thought to myself "why would you intentionally put carcinogens in your body  when I want them out? It didn't make sense, all the while my husband is on the nicotine patch after smoking for41 years..it just doesn't make any sense.That isn't all I saw...I noticed couple walking to their car, the 40 something guy was holding his wife's arm. I then wondered which one just had a chemo treatment. I then saw the woman let go of him and open the car doors. He leaned up to the car next to him from exhaustion,  with his hands on the trunk, he steadied himself and walked to the open door. I started to cry for him, as I know all too well the feeling of coming out of chemo.My heart ached. I have learned so much from this cancer diagnosis, It was a vision that will stay in mind for a long time. I just have to get through this. I want to see Adam graduate and play college football and find his career dream, marry and maybe have a kid, my grand kid.I pray God has his arms wrapped around me, and I pray tomorrow's shot will be okay and raise my blood white cells. i can only do one thing,I have to end on a positive...thank you for my life.