John Powell tells how movies trigger emotions:
Movies take us on emotional journeys, so much so that the psychologist E. S. Tan calls film “an emotion machine.” Unlike moods, emotions require a trigger, and films give us plenty of emotional triggers: joy at the reunion of a mother and a child, fear during a chase, relief after an escape. One way of looking at the combination of film and music is that the film provides a sequence of events to get emotional about, and the music deepens your emotional experience. Even when the action on the screen has no emotional content, music can be used to trigger an emotional response. One study scanned the brains of a group of viewers as they watched an emotionally neutral film clip and found that if fearful or joyful music accompanied the clip, the appropriate emotion centers of the brain were triggered, but there was a minimal emotional response to the film without the music, or to the music without the film. This is very useful to filmmakers, since life (and therefore any film) is full of periods when the visual action is emotionally neutral. By using the right type of music, the director can tip us off that this walk to the shops is going to end tragically or romantically.
(Quoted from page 103 of Why You Love Music by John Powell. Little, Brown and Company, 2016.)