When people stop using alcohol or drugs, they usually describe their physical and emotional state as "unbearable". The withdrawal symptoms are the results of a lack of vitamins, minerals and nutrients caused by substance abuse. This leads to deficiency of amino acids which are used by our brain to build neurotransmitters.
To illustrate what the deficit of neurotransmitters is like, just imagine driving a car with a manual transmission and you are only able to use first gear. It is technically possible to move but it's slow, uncomfortable, and increases fuel consumption.
To get rid of abstinence, return the nervous system to the standard mode and begin the recovery process, it is necessary to go through medical detox procedures.