The process of creating and putting together all thoughts inside my head, seeing new ways the story changes and basically forms itself bit by bit. The joy of writing back and forth dialogue that informs the reader about the character relationships, the world, the setting, the situation, etc. Dealing with certain conflicts and situations that are not so on the nose and can be made subtle through being told a certain way. The way a single idea can form a whole story and that can branch elsewhere and so forth. Seeing your own improvement over time as you read, write and edit more.
Surprising yourself with how well a scene went or how you chose to write something, in a way you would have never thought of years ago. The release of the words just flowing off the page and your fingers dancing all over the keyboard (or sloppy frantic writing all over the pages). Sometimes I have not released that I haven't breathed in a while because I was so caught up with the text. There's an audible "whew" given from me because it feels like a great weight lifted when you can see thought typed out and not in your head anymore, not potentially lost because you didn't write them down yet.
Overall being able to correctly convey what you want, how you want, is a great feeling. The emotional and physical aspect of writing is understated as well.