I'd feel the same if I took down an enemy many times more powerful than me. If I beat up a evil twin or managed to take down the tarrasque many levels earlier than CR would lead us to expect. But when game balance dictates that you're mechanically superior to your opponent, mechanical choices begin to feel hollow. What I mean is that the game is stacked in favor of the players. Just think how often you've experienced a TPK. I'm betting it's significantly less than half the time. It's that knowledge that's hard to reconcile with the power fantasy.