DM here, mine is a bit vague and cheesy but whatever. Favourite part is seeing how excited my friends get when something cool happens! Whether it's pulling out a spicy combat encounter, or they connect the clues of some big plot point, or they pull off something heroic and/or disastrous in game and we all know it's unforgettable.
Being a DM feels a bit like hosting a party (except less overwhelming). It feels like giving gifts to my friends in the form of a fantasy world. It's highly fulfilling, and we end up weaving many joyful memories together.
(Also hello from a fellow Curse of Strahd DM!)