How To Stop Mountain Lion From Logging Out (Shutting Down/Closing Apps)

Have Mountain Lion and annoyed that you are being logged out and your applications are closing?

Follow the steps below to turn this OS X Mountain Lion setting off.

1. Choose Apple menu > System Preferences, click Security & Privacy, and then click General.
2. Click the lock icon to unlock it, and then type an administrator name and password.
3. Click Advanced.
4. Uncheck the box “Log out after ... minutes of inactivity.”

Source: OS X Mountain Lion: Set your Mac to log out when not in use