In my case it was always me simply because its the role I wanted to play and the others wanted to be the players. I loved being the DM or Storyteller or whatever iteration it was for that game. I loved spending my time creating a backstory, NPCs (Non player characters) for the group to interact with, peppering my world with little nuances. What it really boils down to is a creative process that was satisfying to me.
Not to mention I actually love reading the games rules and accessories like Scottie reads technical manuals. I spent hours reading and rereading the rules I could quote just about every little thing.
Its really a collaborative effort though...your players have to be involved but there are tricks to getting them involved if they are reluctant or shy.
Im a gaming dork.
Exactly the same for me. I wanted the role, volunteered for it, and was successful.
I devised a great campaign, have had multiple players play through it, and all enjoyed it. The greatest challenge D&D faced was technology replaced face to face gaming: getting the players together became such a struggle.
Loving this thread, and oh yes I will contribute to it.
I fell into the role as a DM. I started developing a campaign world of my own in about 1982 and have been at it off and on since. My other friends ran (and still run) other games like Traveller, Gamma World, etc. I have always felt to paraphrase Spock from Star Trek II that being a DM for AD&D ". . .was my first, best destiny. . .anything else is a waste of material."
Yes the geek meter just went off the charts!