I honestly think it's the intensity of the endgame. You could be winning like a BOSS, and still end up losing. The great thing is that you never get sit the hell back and say "kk we win" because that's not a reality. I can't describe how many times my team would be winning 30-10 or something, then end up losing.
Usually it's because those nexus pushes are so ******* hard. With a fountain so close to the turret, and endless minions coming in, the winning team is literally FORCED to push and risk dying or let the enemy farm. Either way, both could end up making you lose no matter how much you think your pubstomping. Another great thing is that if you screw up, your ******. The respawns are such that if you mess up a push and end up having your carry killed, then enemy team can flip things around almost instantly.
With all this being said, I can confidently say that this is the reason I play 3v3. In the mere 30 minutes you spend in game, you get to experience all of these directly or indirectly. I'm left to the point where I am literally thinking "holy **** that was intense, lets do that again." After playing over 1000 3v3 matches, these feelings still seem lit.