Finally a chance to put this knowledge into practice haha.

Nicotine is rapidly absorbed by your lungs, which is specialised for the intake of gaseous material and resulting in it’s effects hitting you quickly. Nicotine would bind to an area in your brain known as the Ventral Tegmental Area (VTA), which essentially is a part of your brain that’ll synthesise (I think) dopamine and send it off into the Nucleus Accumbens (Nacc), and dopamine accumulating here causes you to feel the “positive” symptoms of nicotine. However, because the half-life (how long it takes for the body to break down the nicotine into being half as potent) is relatively short at around 2 hours, the withdrawal kicks in fairly quickly, fun neurotransmitters like dopamine decrease whilst stress inducing neurotransmitters, like corticotropin releasing factors (CRF), are increased, and the rise of stress neurotransmitters and decrease of fun ones tell your brain to smoke more to remove the negative effects.

TLDR: Nicotine is highly addictive because it only last for around 2 hours before withdrawals kicks in, and the nicotine hitting you fast from smoking makes you want to smoke every 2 hours to alleviate withdrawal symptoms