Too many reasons to count.  I grew tired of my horrible smoker's cough, always being out of breath, smelling like an ashtray, feeling like an outcast in a society growing increasingly hostile towards smokers, spending tons of money on something I knew was killing me, etc. etc. etc.

I guess I could just simplify that and say I quit to lead a better life.  And yes, life is MUCH better as a non-smoker.