Because you're an addict. I am, too. I still want a smoke after three years being quit. The urges are more seldom and they diminish in strength, but they are still there and likely will be there for the rest of my life.
We are nicotine addicts.
Don't give in. Compare today's urges and desires for a smoke with those that you had your first week being quit. I'm sure today's are lesser. Next month's will be even lesser. But always keep your guard up. Alcoholics know they can never have a drink. Heroin addicts know they can never have another "fix." Nicotine addicts know NOPE ... not one puff ever. We're just one puff away from a pack a day. Keep your quit