In Team Fortress 2 years back I was playing as an Engineer, since nobody wanted to play a defensive role whatsoever despite us trying to capture a point while defending our own. The match was going on for hours, the longest I've ever played, and every time there was a push the opposite team would come through and just about manage to fend the attackers off.
It got tiring after a while so I decided to go suicidal. Still an Engineer I sprinted into what had basically become no-man's land, shotgun in hand, at a time when only a few people were milling about. Somehow, luckily, I killed those few people off.
"Now what?" I asked myself, fully expecting to have died by now. "I know, I'll just place down a Sentry here for funsies and see what happens..."
What happened was a rush of other Engineers suddenly following my example, and laying a belt of rapidly-progressing Sentries that soon made the push for the enemy's base all the easier. There was lots of shouting and screaming from different voices "Sentry down on the left! Supercharged Heavy spotted!", it was the most I'd ever seen people band together like that in a game, and after several hours we managed to push through the the enemy's point and, still as an Engineer, I was there, wrenching the crap out of whoever dared to cross the path of my comrades, on the point as the last few seconds ticked away and, finally, BLU team won.
That was the happiest I've ever felt in a game, before or since.