Love is so hard to explain. There are some who believe it's simply an emotion—a sort of force field, like gravity, that draws people toward one another. Others consider it something you decide—a choice to hang around, to take care of another human being, and work through the tough things. Is it something that occurs to you, then, or something you do every day of your life? Frankly, like with love itself, it's not so clear. Falling in love has that raw, dizzying thrill about it. It feels so easy and electric that you hardly have any idea what hit you. One minute you meet someone who lights up bits of you you never knew you had, and before you know it you're thinking that love is simply that feeling. Your heart thumps in your chest, your palms get slightly moist, and for one moment life is more colorful. According to some experts, it's simply because of the mix of chemicals—dopamine, oxytocin, serotonin—coursing through your brain and making you feel high and strangely...