Indeed he summons them to crave that spiritual food. It is a strong word. Like a newborn baby craves the breast, cries for it, even wails for it, we Christians are to crave this milk. It is not as though Peter is saying that these Christians are spiritual children who can handle nothing but milk. They should be ready for meat but they are not.
Milk is simply the appropriate food for the newborn and the young.
Milk is here a symbol of spiritual nourishment, especially the nourishment that is brought by the Word of God, to which reference was just made at the end of chapter 1. It is interesting that this picture of milk as spiritual nourishment was so powerful that, according to the Apostolic Tradition , a third-century work by Hippolytus, at least in some parts of the Christian church, after baptism a cup of milk mixed with honey was given to the new Christian along with the bread and wine of the Lord's Supper. Life had begun, growth in that life, maturing to adulthood was now called for. The idea of growing up in salvation makes sense because Peter is speaking of salvation not as a synonym for the forgiveness of sins or justification, but for the entire process of renewal and restoration of life that culminates only when Jesus comes again. And, then, the third part of the argument, in v. 3.