Why must you eat protein in order to make protein?

If you eat protein food you will build up and make protein.

The protein that you eat goes toward making protein in your body for different uses.  For example:
the body uses protein to make hemoglobin, which is a part of a red blood cell that carries oxygen to parts of your body.

And some proteins help build cardiac muscle, which is your heart.  

So, you must eat protein so that your body can function properly.  If your body did not produce hemoglobin, the red blood cells could not function properly, and you would not get enough oxygen throughout your body.

Hope this helps :)