From Mindhacks:

Psychiatrist Donatella Marazziti and her colleagues measured levels of a protein that transports the neurotransmitter serotonin in the blood of 20 people who had recently fallen madly in love, 20 people with obsessive compulsive disorder (OCD) and 20 healthy comparison participants. … She found that the group of patients with OCD and the recently love-struck were no different in terms of the serotonin transporter protein, suggesting the brain began to function markedly differently as love blossomed.

If you enjoy reading about the chemistry that happens in our brains when we love and feel concern for others, abstracting the most meaningful emotions and experience of human existence into soulless base molecules (like I do!), may I also suggest the great blog Hug the Monkey, which follows research and study of oxytocin, a hormone that influences—or, rather, is—our feelings of comfort and love.