Most Helpful Guy
Chemistry, unfortunately. Those early feelings of love are fuelled by a cocktail of chemicals in the brain - dopamine, norepinephidrine, phenylethylamine, serotonin... essentially you are feeling the effects of amphetamines, stimulants, and painkillers all at the same time. And these are hard drugs, and addictive drugs. So breaking up means you are kicking an amphetamine addiction, a stimulant addiction, and a painkiller addiction all at once. The painkiller part is why the feeling of breaking up can actually cause very real physical pain. It can actually hurt, really physically hurt. And it's because you've been dosed on painkillers and suddenly those painkiller chemicals have been cut off.
You will get over it eventually, everybody does. It will hurt while you go through it. A lot. But with a good support network, and talking out your problems with a sympathetic ear, you will get past it and things will turn around.