German Shepherd Questions...?

Okay I have a GSD. I am a new dog owner. I know people say dont get it for 1st dog but i already did and I am not returnin him so dont say return him. Why do people say that? I am curious. Okay here is my real question... My dog does not sit when I say sit or come when I say come. He knows the commands, but he never does it unless I have his full attention. It can get frustrating. He also never does the command unless he sees that I have a treat in my hand. My GSD is 10 weeks old and a Czech breed. thaNX