Originally Posted by
Buttons.
^ Because they sympathise with the predicaments the people in the film are facing and they feel what they would feel in that situation, for example if I were to watch Beauty and the Beast there's a bit where she thinks she has lost the person she loves most and I can sympathise with this despite having never lost a partner (but having lost other members of my family) so I feel sad for the character and cry, but then equally I smile when things turn out alright for them.