Illustrator default Preferences are saved in a file on your system that is launched when you start Illustrator. This is why every time you restart Illustrator, you’ll see the workspace in the same configuration as you last left it. The Prefs file controls other options too, and if you’re having problems with Illustrator, sometimes it helps to reset the preferences and restore the program to it’s default settings.
Instructions for resetting illustrator defaults:
Mac OS: Hold Command+Option+Shift as you launch Illustrator.
Windows: Hold Control+Alt+Shift as you launch Illustrator.
You can also manually delete or rename the preferences file on your system and Illustrator will create a new file when it’s restarted.
For directions on locating the Illustrator preferences file on your system, visit this link to Adobe’s Illustrator help site. The chart only goes up to version CS5, but the location has remained the same for Illustrator CS6. Illustrator generates a preferences file (named Adobe Illustrator Prefs or AiPrefs) after you customize your settings. If you haven’t changed anything from the default settings, you will not see the file on your system until you do so.