Do you mean "Why did you get into gaming?" or "What did you get into gaming for?"
In any case, I started out with the NES my parents had in the day with a black box full of cartridges. Mario, Duck Hunt, Zelda... all that good stuff. Though I understood very little of what I was actually doing with most games, I spend hours on that thing.
Then, when I was about 8 my friend got a classic Xbox with *drumroll* Halo: Combat Evolved. And although I was hesitant playing with him, I did enjoy watching him play. And after some time I started playing co-op with him. Then he got Halo 2 and I got my own classic Xbox with Halo 2. But it wasn't untill late Halo 3 where I got my own Xbox 360 with an internet connection (before this I always played online as a guest at my friends house, and with my guest account I always had the top of the scoreboard. Which was pretty funny and raised some eyebrows on the enemy team on occasion).
Anyway, late Halo 3 is where I got into online gaming on the Xbox. I did play some cracked versions of COD 4 and a legit version of MW2 for a bit on the PC before Halo 3.
I loved the idea of actually competing with actual people online and I still love that idea to this day.