For one thing, it causes serotonin levels in the brain to drop, which may lead people to obsess about their lover. (The levels of serotonin, a chemical produced by the body, are also low in people who have obsessive-compulsive disorder.) Next, it ramps up production of the stress hormone cortisol, leading to slightly higher blood pressure and possibly loss of sleep. Finally, a scientist at the University of London has found that when people look at their new loves, the neural circuits that are usually in charge of social judgment are suppressed.
All in all, love, kind of leaves you obsessive, stressed, and blind. And we can't get enough of it.