Your brain is in withdrawal from the release of dopamine your ex gave you. In fact, recently broken-up singles show activity in the same part of the brain as a drug user. Dopamine is the feel-good chemical the leaves you wanting more of whatever stimulus gave you the dopamine in the first place. Cravings encourage you to get more of what your brain wants, and in this situation, your brain wants your ex. Understanding this can help you break the cycle.