Idk it's like a release of emotions I think. The music sort of puts your feelings into the track or cocoons them through displacement with the sound so it's like you've been purged of your feelings and that gives a sort of relaxing feeling. (Sorry if that makes no sense, literally fail of a sentence.)
Explains why I only ever cared to listen to the most extreme of death metal during middle school- to release anger, appearing from absolutely nowhere, which couldn't be released elsewhere without someone dying. Also I suppose I could relate to the lyrics, and so it felt as if I wasn't alone with such emotions. Nowadays it just gives me a rush of energy that I can use to exercise with, like the video below. The only metal I can use as "listening music" nowadays is .00004 the speed and intensity of normal metal and is more akin to prog rock than stereotypical metal.