A physiologic cause would be the neurotransmitter dopamine, but that is probably an oversimplification as addiction goes much deeper than that. Basically dopamine is released whenever you do something that is rewarding. It is tied to almost every action that promotes the survival of the human species as well as yourself. Think: sex, social interaction, food, love, etc. Now when you use (most) drugs you are artificially creating a flood of dopamine that your brain interrupts as a reward. It then associates the drug with other actions like food, sex, etc, only on a much stronger level since the amount of dopamine released during eating or sex is much less than the amount when you get high. This is one of the many reasons why it is so hard to stop. Your brain still associates the drug with something necessary for your survival (as well as feeling good).
This is also why many people need to hit a real rock bottom to finally quit. Once you hit bottom your brain re-associates the drug with your newfound shitty situation.