Well, reactions do occur in the brain. Neurons transmit signals to each other using complex algorithms and the like. But if you generalize this process, it's called "psychology." Or did you mean hormones? That's also a factor; they will affect brain function, undoubtedly. For example, testosterone increases aggression. But hormones affect a person's overall psychological state, not specific thoughts, including thoughts about love for a specific person.
Or did you have something else in mind?