We start with milk, because it is all the body can handle at this early stage. Babies desire milk, and likewise, we need to desire our milk as a Christian. Our milk as Christians is the Word. It is what makes us grow.
Everybody needs food to grow. If you fill your spirit with junk like dirty music, then your spirit will just become dirty and polluted. On the other hand, if you fill your spirit with the word of God, then your spirit will grow healthily. You will be a firm Christian who will be able to overcome all obstacles that comes your way.