Over at Dreams in the Lich House, Beedo laments* that his players offed his big bad dragon so easily in the end, thanks to some rather good role playing decisions.
I have been on that side of the DM screen (although I don't know that Beedo uses one) and once saw an advanced Dragon Turtle waxed by a group of characters in a manner reminiscent of a banana in a blender, before she could take them to the (steam) cleaners. In hindsight, I didn't handle that well.
It's difficult to create interesting, challenging encounters regularly for your players and when they blow through a carefully crafted encounter or are completely uninterested in the story arc you've spent time crafting; it's easy to get a little down on your efforts.
Bad DMs fall into an us vs them mentality with their players. And having played in a couple of the infamous killer dungeons in my earlier days, there's a reason I haven't kept in touch with those DMs. They weren't fun.
A good DM keeps their perspective and is flexible to allow the players to explore the his or her world in their own fashion. It's game and when the players have enjoyed their session, you as the DM have won.
But don't rest on your laurels, Sisyphus, you have the next session to prepare for.
*Although from the tone of the article I suspect he's kind of proud of them.