It is a mental health issue. Nicotine affects neurotransmitter release, for example in the reward (dopamine) pathway. Nicotine binds to the acetylcholine receptors on neurons, a signal is sent, dopamine is released into the synapse, then binds to the receiving neuron, which now continues the signal. Voila, the dopamine pathway. Pleasure! (I believe there also may be nicotine effects in the serotonin system.) Sometimes people with certain neurotransmitter imbalances may attempt to self medicate by smoking.