system
Given the following post, respond as if you're commenting on Reddit:
human
What makes a substance addictive and why does the brain and/or body think that it's required or that it needs more?
gpt
Hey.  In basic terms, the brain is set up to encourage survival-promoting behaviors.  So, sex feels good.  Eating feels good.  Affiliating with other members of the species feels good (usually).  We have a reward circuit that is activated by these survival-promoting behaviors.  So your brain goes, "Do that again!"  

When you take certain drugs, this reward circuit is overstimulated.  The stimulation is beyond anything that "normal" survival-promoting behaviors elicit.  So your brain goes, "HOLY SHIT, DO THAT AGAIN.  AND DON'T DO THE OTHER STUFF THAT ISN'T NEARLY THAT GOOD."  Your brain doesn't know that this stuff isn't survival-promoting and, in fact, will eventually destroy you.  The brain is doing what it is supposed to do = keep doing the stuff that activates the reward circuit.  

That's a pretty simplified explanation.  There are many other factors and theories about addiction.  There are theories that posit that some addictive behaviors are the result of avoiding withdrawal, some are avoidance of emotional pain or anxiety, some revolve around difference in endogenous levels of neurotransmitters among individuals, self-medication of psychological problems, etc.  But when you are talking about that desperate desire to repeat a negative behavior in the face of life-destroying consequences, there is reward going on in the brain.