Why do I have to keep inputting my email password on my macbook air?
I'm not understanding why this is happening with you. Well basically, you can say your mac is "forgetting" your Apple ID / email or whatever. If this situation is related to an Apple ID, using The Mac App Store, logout and login and tick the "Don't Forget"

You can use Keychain Access! You can modify which applications will automatically log you into your email while starting the application.