I'm curious as to why is quitting smoking so hard?

Quitting somking: The substances in a cigarette are addicting similar to alcohol or opiates or medication such as valium. These substances have complicated effects on the chemistry of the brain. The brain becomes used to them. The chemicals take over some of the actions of the brain so when you quit the brain feels a need for them. You feel bad. It takes time to adapt.

Many reasons: Nicotine activates primitive pathways in the brain associated 'reward'' and 'safe/secure'. This is the same primitive signal that we associate with other good feelings; food when hungry, new love, or even sex activate the same signal. Taking that signal away is very unpleasant. Stopping nicotine leads to the opposite, loss of a sense of security (anxiety) and eliminates the 'reward' signal.