I originally started because my friend (now roommate) wanted to quit, and he only had a few left, so I said, I'll smoke half because then you'll have fewer to smoke and it'll be easier. HAH! It was easy for him already, he told me he's got no urges at all. Not so for me. After a few hours without a cigarette (unless I'm asleep), I get angry and stressed out. It does hit me pretty hard. I also know the downsides of smoking, but my brain demands nicotine, and it tries to murder me if I don't get it any.
When I don't smoke, basically, I get VERY apprehensive, stressed, and angry, and it's all made worse by the fact that I'm full of anxiety most of the time anyway. Smoking just makes me calm down for a bit, and I can deal with everything a bit better. And a majority of people CAN'T get over it easily enough. In fact, something like 9 in 10 can't quit smoking. And on top of that, I have a pretty addictive personality anyway.
On a side note, I now have 4.9 packs sitting on my desk next to me, so I should be good for a while.