When we get an unexpected reward (for example, a glimpse of our father's meager affection when we're young), dopamine is released and we feel euphoric. The dopamine then stores that conditioned response and associates it with the reward. When that reward (or affection, in this case) is taken away from us, the dopamine is decreased. This will trigger a phenomenon of craving, and the stored association will then trigger behaviors that will again lead to that reward. (For example, seeking validation from the one person we've deemed responsible to right the wrongs of our father's mistakes…)
When the dopamine in our brains is lessened, in addition to the inability to satiate the craving (he doesn't call, he stands you up, he never comes home, etc), comes dysphoria – the opposite of euphoria – and we literally go into a state of withdrawal, not unlike kicking a bad drug habit. Some characteristics of heroin withdrawal, for example, are anxiety, depression, malaise, loss of appetite, nausea, sleep difficulties, cramp-like pains and a general feeling of heaviness. Add some tears and a long, dark night curled into the fetal position and you have yourself a good, old-fashioned heartbreak.
The real question is: does all this knowledge of the inner workings of our minds make the pain any easier to deal with? You bet your lonely ass it doesn't.
