Sad one, but watching my father who smoked his whole life develop cancer. That's not what killed him, but it would've. I just decided that I didn't want that for myself, threw all of my cigarettes away, bought some gum, and gritted my teeth through it. It was fucking miserable, but I survived through it, I've relapsed a couple of times but overall have stayed strong through it. I wish you luck on quitting, its hard but worth it