It is funny for me to think of love as a drug, but I was reading this article on howstuffworks.com which takes a look at the chemical responses in the brain during relationships. Being in love literally releases hormones that give you a high. Oxytocin, Vasopressin, and endorphins seem to help create emotional bonds and instill a general sense of well being. When the relationship stops, the hormones stop getting produced. While we are emotionally most vulnerable our bodies start going through withdrawal. It isn't pleasant to live through, but the science behind it is pretty cool.