Most Helpful Guy
It's because you are essentially going through drug withdrawal. The chemicals released during the early stages of a relationship are not 'real love', but feel like it. Dopamine, norepinephidrine, phenyltethylamine and serotonin are all released by the brain during that phase of a relationship. So you are basically 'love high' on amphetamines, stimulants, and painkillers. These 'love chemicals' are hard drugs, and are addictive drugs. Breaking up causes that chemical release to stop. so you are going through withdrawals of amphetamines, stimulants, and painkillers... meaning you can actually start feeling physical pain, and you feel depressed and lethargic. It is a terrible, terrible feeling.
But, almost everyone in the world has gone through it. And you will get through and feel better again. Each time it happens, your body gets better at coping. You will find other people. You will experience those highs again. Your brain chemistry will normalize again in the future. You are in pain because of biochemistry, not because of anything you've done wrong. It is your body reacting against your will. And it will calm down in the future, so you just have to hold on and bear it. It will pass.1