iPhone app store problem

Why does it happened that every time I want to download an app, it asks me for my user ID and my password, why do I have to re-enter it every time? I know I had to re-enter the password but now I always have to do the same with my user ID. Can someone help me out, please

I have the iPhone 4s, with the newest version, and my iTunes on my computer is the latest version too.

Thanks