How in the hell can the meat be so dry when you just saw half a gallon of brown juice ooze out of it? Well, that's the problem -- the juice is now on your plate instead of in the meat. You didn't let the meat "rest" before cutting into it.
Why That's a Bad Idea
What, you thought that those warnings to let the food sit for five minutes before eating was just to let it cool so you wouldn't burn your mouth like a dumbass? There's a bigger reason: Once the meat heats up during the actual cooking process, the juices inside start to "boil" from the inside outward. What's actually happening is that as the meat cooks, the muscle cells contract, and the juices get squeezed out, like ringing out a dishrag. It's why you can put a piece of meat into the oven, dry, and it develops a small puddle by the time it's done -- that's not all melted fat.
Sometimes, it's the tears produced by our soul-crushing lives.
When you pull it off of the heat, it will continue to cook for a few minutes. The residual heat from the outside of the meat will try to gain equilibrium with the cooler inside, so the center heats up while the outside cools down. A roast will typically rise another 10 degrees in temperature as it rests. That's important because as the meat rests, the equilibrium process will reach a peak before finally cooling as a whole, and at that point the muscle cells start to relax, turning into a sort of sponge and soaking up the liquid that was squeezed out earlier. Much like your bear costume at a Craigslist swap meet.