If you are not growing, you are dying! Peter begins with a 'therefore' (or 'So...' in the ESV); based on what we have been told in 1 Peter 1:3-25, there are things we need to do!
He begins with a list of five factors that will kill growth: malice, deceit, hypocrisy, envy and slander. He then encourages us to long for the "pure spiritual milk" that will enable us to grow.