You're like a couple of people I know: they can start and stop at will. I think it all has something to do with the addiction part of the brain. Some people can get additicted to substances easily, some normally, and some not at all.
I only have 1 really bad habit, and that's smoking. I don't drink, do drugs, eat red meat, only have fried foods about once a month, and the ONLY reason smokers have become pariahs is due to the hype from the media. Smoking really isn't any worse for you than eating fast food 3 times a day, or frying everything (aka poor diet).
I smoke about a pack a day (large) a lot less if I'm working somewhere where I can't smoke inside. I kind of like this because if I didn't stop for a smoke once in a while, I'd never take a break......
For those that think it's easy to quit: The addiction researchers have determined that nicotine addiction is worse than heroin......just saying is all.....