Why is my macbook asking me to use my password when opening an app, or wanting to access my keychain account?

So when i open an app it says "(app name)" was purchased using the App Store on another computer. To use "(App)" on this computer, sign in using the Apple ID and password used to purchase it.

But the thing is that this is the computer i used when i bought these apps, and now when trying to put new music on my iPhone, i have to erase and sync everything because it says its already being used on another iTunes library even though I've been using the same library since i got my phone. Its like my macbook has Alzheimers and can't remember me or itself. any help?