"...When shown a picture of their romantic partner, their brain activity pattern was markedly different from when they looked at a picture of a close friend, reported neurobiologists Andreas Bartels and Semir Zeki of University College London. The pictures showed that the experience of romantic attraction activated those pockets of the brain with a high concentration of receptors for dopamine, the chemical messenger closely tied to states of euphoria, craving and addiction. Biologists have linked high levels of dopamine and a related agent, norepinephrine, to heightened attention and short-term memory, hyperactivity, sleeplessness and goal-oriented behavior. When they're first captivated, Helen Fisher, an anthropologist at Rutgers University in New Jersey, argues, couples often show the signs of surging dopamine: increased energy, less need for sleep or food, focused attention and exquisite delight in smallest details of this novel relationship.
"Bartels and Zeki compared their MRI images to brain scans taken from people in different emotional states, including sexual arousal, feelings of happiness and cocaine-induced euphoria. The pattern for romantic love was unique. But there was some overlap with and close proximity to other positive states..."
— Benedict Carey, Los Angeles Times, 2002

I can sum it up in one word: Bliss.