She didn't want Persephone (Demeter's daughter) to be in a gloomy place but Hades and Demeter strike a deal "If you let me marry your daughter i'll give her back for 6 months and after the 6 months are over you give her to me and we go back and forth." Hades said. Demeter agreed and also that's why we have seasons
I really hope I helped
-Ciel