What are the reasons that people claim that cigarettes are more addictive than heroin?

Depends: On the person. Both are highly addictive and very hard to break.

What they mean is ..: The chemicals that are most likely to cause addiction are those that reach the brain quickly, greatly stimulate the "pleasure center", and then leave relatively quickly. Although nicotine is a more addictable chemical, heroin addiction is worse for you in every way. It is very easy to become physically and psychologically dependent on heroin.