There’s all kinds of reasons that you fall in love with one person rather than another: Timing is important. Proximity is important. Mystery is important. You fall in love with somebody who’s somewhat mysterious, in part because mystery elevates dopamine in the brain, probably pushes you over that threshold to fall in love.

Source: "Why We Love, Why We Cheat." TED2006. Feb. 2006. Conference Presentation. Additional Information: With this quote, anthropologist Helen Fisher explains the psychology and biochemistry of love: it's a delicate balance of timing (when in your life you meet a potential love interest), proximity (your physical closeness to this person), and mystery (when you don't entirely understand the person's behavior or part of them remains unknown to you). Fisher explains that the mystery factor in particular positively affects the brain's levels of dopamine, a neurotransmitter that is correlated with the feeling of "falling in love." Watch the full TEDTalk here: Helen Fisher - Why We Love, Why We Cheat