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.