The Why Am I Here easter egg can be found on Beaver Creek on Halo 2. Walk up to where the Sniper Rifle spawns, then head for the end of the path. Once you have made it to the end of the path, jump past the dead end. On the right side, there should be a place that you can jump onto. Then, there are some rocks that you must push down. Where the big one used to be, there should be "Why Am I Here" and some initials. If you still can't see it, move back a bit. Try zooming in to see it. The initials are "S.M."
In the Rooster Teeth series Red vs Blue, the first episode is called "Why are we here?"