Several magical spells require the use of certain emotions when casting them. The Patronus charm, for example, requires the caster to concentrate on a happy memory. (Force of will, under extenuating circumstances, helps a lot. An example of this is when Harry is able to conjure a corporeal Patronus when Sirius is in danger of being administered the Dementor's Kiss.) Another example is the Cruciatus Curse, which causes immense pain; as Harry discovers during a duel with Bellatrix Lestrange, effective use of this forbidden dark magic requires sadistic desires.