Why we have to damage our muscles to build them?
Muscles are developed as required by the surrounding environment. This means your body is constantly physically stimulated to grow muscles according to the external forces it experiences (e.g. gravity). 

When you train your body, e.g. with weights, you are applying an unusual amount of pressure to your muscle cells, damaging them in the process. In the process of healing your muscle cells, your body will strengthen them in order to adapt to the new physical stimuli, but only when a vast amount of required elements through nutrition is available (proteins).

Quite an impressive feature, right. :-)

I know this from researching personal training/weight training.