A lot of people (including myself) have been having issues with the OS X launcher. After downloading and running the .dmg, the launcher will say it is updating, and needs to download runtime. When runtime gets to 59.90 MB out of 59.92 MB downloaded, it will start over at which point it will attempt to download runtime again, doing so until it's attempted to download it 3 times, then it will give the error, "Unable to update the minecraft native launcher." and then it will close.
Restarting the computer, and removing and redownloading the OS X launcher does nothing.