At How Stuff works, found this interesting article titled: "How Love Works". According to this article,
People often tend to fall in love with people in whom they see their parents, siblings. With whom they have been associated from childhood.
Eye contacts and looking deep into the eyes, has an effect in two people falling in love.
Love, relationship, Lust, Passionate Love, Infidelity etc is explained entirely as a chemical phenomena. Get to know your adrenaline, dopamine, norepinephrine, phenylethylamine, etc.