Quitting smoking is generally considered very hard. Anecdotally, most attempts seem to fail. Malcolm X is said to have said that quitting Heroin is not easy, but smoking is the real challenge.
I can't speak to the biochemistry of it all, but from a purely anecdotal-behavioral perspective, it is not obvious why quitting smoking should be particularly hard. I can see one big argument for why it should be easy and one for why it should be hard.
1. Should be easy. Beyond the addiction itself, there's little reason to smoke. It is not hard to see why people would consume alcohol or heroin: They're psychoactive, in ways that are often experienced as extremely pleasant. Strictly speaking, nicotine's psychoactive, too, but, really, that's negligible. So, in that sense, smoking is the stupidest addiction there is: You don't even get a high out of it. Why not stop altogether?
2. Should be hard. Being addicted to heroin or alcohol fucks up your life. You probably won't be able to hold down a job, and mess up your personal relationships as well. (Yes, there are high-functioning alcoholics. This is one of those cases in which the existence of the term tells you that it's been invented to describe an exception. Nobody would come up with the term "low-functioning alcoholics" because that's just, you know, regular alcoholics.) In contrast, the near-term consequences of smoking are minor (smell, yellow teeth, shortness of breath), while the biggie (lung cancer) is far into the future, and you, like everyone else, are a time discounter.
So, no major reasons to continue, but not that many reasons to stop either. Should be a wash in those terms, right? And yet quitting smoking is considered unusually hard. Let me suggest that this is a variant of the phenomenon "bad is stonger than good" (low quality pdf). That is, other drugs give you a better reason to continue (good), but they also give you a better reason to quit (bad). Bad gets a higher weight than Good, so if both are stronger, people feel less of a motivation to kick the low-bad, low-good addiction than the one that's high on both. Why go through all the trouble when you're not hurting yourself all that much?