You killed the baby reaper or won some other big fight? Yes it's only a video game but still after my first playthrough I felt a real sense of accomplishment.
Anyway: my Shepard + cain + big ass badguy looking down at me?
I get the same feeling of having done something significant after polishing off end of game baddies in pretty much any video game - nothing wrong with that at all
Honestly, I was on Pinnacle Station for ME1 because I created a character in the likeness of my girlfriend. She was over, and I was showing her how much her character kicked ***. I was singing a song, can't remember which at the moment. I know it was a song about staying alive though.
Man was I wrecking those turians in The First Contact simulation. They had more body bags to go home in than there was seconds spent fighting them. Also, I got an applause from Ashley and Tali when we finished the mission for not only keeping us all alive, but for my beautiful voice.
I was in the habit of muttering "Headshot... BOOM" when I killed big things with a headshot (as an Infiltrator, that means almost every kill), Then I killed my first YMIR with a headshot from way too close, and before I was done saying my line, the thing went nuclear in my face, so it sounded like "Headshot Boooooh crap"