The comparison clearly comes from Peter’s earlier analogy of being “born again” (1 Peter 1:3, 1 Peter 1:23). As any mother can testify, a healthy newborn has a natural and frequent longing for milk. Peter’s readers, newborns as they are per 1 Peter 1:3, 1 Peter 1:23, ought equally to crave drink.
2 Like newborn infants, long for the pure spiritual milk, that by it you may grow up into salvation