what is it about drugs that make them so addictive?

what is it about our bodies and our minds that cause someone to become addicted to something such as cocaine, heroin, or meth. what is it about our brains that cause us to become addicted to alcohol? and why do our bodies go through withdrawals without these things once they become addicted? obviously because they are used to it, and need their fix, but what is it about our brains exactly that cause us to become addicted? [hope that doesn’t sound like a stupid question]

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