I have just updated my Xcode to 6.0.1 from Mac App Store, and my OS version is OS X 10.9.5. The problem is that the iOS 8 Simulator is not loading, but the iOS 7 simulator is working fine.
Every time I try to boot an iOS 8 simulator device, I get an error message:
"Unable to boot iOS Simulator"
The main cause of this error is that DYLD_INSERT_LIBRARIES is set. I suggest you unset DYLD_INSERT_LIBRARIES (likely in /etc/launchd.conf) or otherwise uninstall the product that set it, and then reboot.
OS X Yosemite has a software change to deal with this situation, so if you need the product that was setting DYLD_INSERT_LIBRARIES, I suggest you update to OS X Yosemite for your development.
Go into Window->Devices. Find the simulator that's not working, right click and delete it. Then click on the "+" button at the bottom and you can add it back in.
I had to delete a few of them, and this only takes a minute.
来源：https://stackoverflow.com/questions/25987302/unable-to-boot-ios-8-simulator