I've been GMing for quite some time. I started shortly after I started gaming.
In the distant future of the Year 2000! (wait, that's not the future anymore, is it?), the Third Edition of Dungeons & Dragons was released to much fanfare. Lots of people were eager to play. There was almost a constant buzz of "Let's play D&D!" at the local gaming store.
But one thing that any game of D&D needs is a Dungeon Master. But it wasn't easy being a DM in those days. For one thing, the three core ruilebooks were given a staggered release. While everyone could readily buy a Player's Handbook, the Dungeon Master's Guide and Monster Manual had yet to be released. The first printing of the PHB came with a "Y2K Survival Guide", which consisted of a brief overview/preview of the rules from those books, so you could cobble something together.
Also, thanks to the Open Gaming License, a lot of (too many, really) publishers were releasing adventure content, among other things. So while everyone was picking up PHBs and third party supplements, I was one of the few people picking up adventure modules. Everyone had Dragon magazine, and I had Dungeon magazine.
So amid the steady buzz of "Let's play D&D," there was a vanishingly small number of people who were willing to be the DM. But it turns out that I was one of them. (I commented earlier this month about the "older cousin model" and how DM/GMs get short shrift in spite of how essential they are to the growth of the hobby).
This started with simple willingness, rather than enthusiasm. And there was a very long time that I was completely dependent upon adventure modules before I had the confidence to write my own material.