The catalyst that made me decide to quit smoking is quite a silly one (my 'reasons', however, are the sensible ones that most people have in common - better health, longer life, more money, etc.). I've tried quitting twice before, both times with the help of therapeutic nicotine - patches the first time, gum the second - with no success on either occasion. This time, however, it was a simple little thing that flipped a switch in my head and suddenly made me realise what I was doing to myself. It shocked me so much that I quit on the spot, cold turkey.
I realised that I had completely lost my sense of smell. I couldn't smell food, old books, cut grass, anything. The scariest part was that I hadn't even noticed until one day my mother showed me some potted herbs she had bought in a garden centre, and crushed some leaves to demonstrate how nice they smelled. Immediately, I said 'F*ck these things,' and threw the remaining ten cigarettes into the bin.
I never took notice of scents before, but over the past two weeks, I have completely recovered that sense and am rediscovering what I took for granted before.