What kind of fun is there in being a game master? Some see it as a work, but you need to find what you think is rewarding, and have a group that enjoys giving that reward.
When prepping, you find yourself thinking of how it will end, and paint pictures, perhaps giggle while doing all the prep work needed. Science has shown that even if it doesn't happen, you can still it because of the anticipation of what could happen.
Old roleplaying gamers tend to forget this, but there is a magic feeling in discover what a roleplaying game really is, as the new game master (and players) steps into the unknown and tries to understand how everything works. Everything is new and exciting, and that feeling shouldn't be overlooked.
Designing a fun challenge in, for example, D&D is rewarding in itself. To find out different combinations of powers between monsters, or trying different environment, and then having the players face them.
Build up something beautiful for the characters, then destroy it and thrive in your players' desperate act to hold it together. Just be sure that your players enjoy that feeling, otherwise it's just a pleasure for your own sake.
It's true that you cannot reach immersion because it takes deep commitment over a period of time, but on the other hand you get to play so many different characters that you perhaps wouldn't had time to try otherwise.
When that spark lits in the players eyes, and they can't stop themselves to gasp, chatter or act, that energy is fed back to you.
When everything clicks between you and your players, and you all immerse in the story, you're feeling the adventure is playing itself, or you cannot stop contributing to each other's ideas. When time and place seems to disappear, then you're in the flow.
Some people love to create worlds, and love reading about those worlds in roleplaying books.
All the cool stuff you're reading about, in this game or others, if it's themes, cool characters, styles, genres - all that is yours to include in the scenario.
I swear to you, that there is a guy that is touring around on conventions to talk to others about his character. Just in the same matter, some game masters love to talk about what they created on their sessions.
Have a huge pre-written plot? Having players that enjoys that feeds back to your engagement as a game master.
Just like a fishing trip, or a vacation, taking a imaginative tour with others is something that makes the group bond.
You're not only responsible for knowing the setting, the rules, but also knowing the players and create a good group consensus. The people you play with are usually your friends after a while, even if it's a one-shot at a convention.
Cannot wait to try new houserules? A new setting? New narrative techniques? Seeing how it all work together and how the players respond to it makes it all worth your while.
Everything you learn about game mastery, is something that you can share with others. Whatever response you get back, may it be on blogs (ex. www.gnomestew.com), forums (ex. www.rollspel.nu), or people buying your book (ex. Play Unsafe), it's enough to get you going in the role of game master.
Tinkering with houserules or tweaking certain mechanics is a joy in itself, especially when it turns out to work as intended. And if people would start using it, that's the jackpot!