I keep having to restore my iPod

About once a week I've been having to restore my ipod! When I open iTunes it gives me the message when you first hooked up your iPod, where you give it its name. I click ok, and then it shows that most of the 'space' is taken up, and that my entire library can not fit. This 'space' is the size of my library only my ipod doesn't recognize it as my library. I then restore every thing and its fine. But why does this keep happening? It takes forever to resync using usb!