Opioids basically ARE dopamine in many ways. So what happens when you abuse them is that your body stops making it's own, and it takes awhile after cessation for the process of making your own ... to bounce back. So when you quit the dope you're left with NO dopamine basically.
But it's worse than that because the opposite happens with your adrenaline (a rough, non-precise term but gets the idea across) ... you're giving yourself a CNS depressant, which inherently makes you slow and tired. So your body compensates with over-production of 'adrenaline' to keep you awake and functioning. Over time that just becomes automatic and again, there's a bounce-back period after cessation.
IOW you're stuck with 0 'feel-good' chemicals (and don't forget, dopamine is also your natural pain-killing chemical which means everything HURTS, esp. your back), and an overabundance of 'panic' chemicals.
It is ... a pretty brutal combination of effects 
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