Why does my chicken breast become so dry?
It's my opinion that the "proper temperature" is a number set by lawyers, not by chefs. The government standard for a roast bird is 180°F (83°C)! Are you kidding me? HTST pasteurization is 161°F (72°C) for 20 seconds, but my turkey has to get to 180°F for safe human consumtion? What the hell kind of bacteria do they think live in there?

165°F (74°C) is a common number, and a pretty decent one, though I always take mine out before that for the reason below.

You need to remember that the internal temperature of the bird will continue to rise for a while after you take it out of the oven: the heat on the outside is still migrating inward. If you wait to take it out until the internal temperature is the "right" temperature, by the time it peaks, it'll be 10°F (~5°C) HIGHER than the right temperature, and that's in the dry zone. We're talking 190°F (88°C) thanksgiving football. Blech.

Dark meat almost always handles being overcooked better than lighter meat. The breast is the hardest part of the turkey to cook correctly. If you haggle with the temperature, and your white meat is still on the dry side, you might try brining the bird for 24 hours or so. It makes a big difference in terms of juiciness.