Shakespeare's Sonnets: Why is my verse so barren of new pride?

Why is my verse so barren of new pride?So far from variation or quick change?Why with the time do I not glance asideTo new-found methods, and to compounds strange?Why write I still all one, ever the same,And keep invention in a noted weed,That every word doth almost feal my name,Showing their birth, and where they did proceed?O know, sweet love, I always write of you,And you and love are still my argument:So all my best is dressing old words new,Spending again what is already spent: For as the sun is daily new and old, So is my love still telling what is told.