The body is an amazing mechanism. If it doesn't need to be stronger, it won't use the protein in your diet to rebuild muscle tissue.

However, if you place a workload on your muscles (resistance training), you are now forcing your body to adapt to a new stimulus. If it regularly has to lift weight, then it will compensate by making your muscle fibers grow in the area being worked (note: it is a VERY gradual process ...)

If you are not lifting weight, and there is no workload to adapt to, then your body/muscles will stay the way they are.

Once you have it, you have to maintain it too. It's all a case of use it or lose it.