Why is my verse so barren of new pride (Sonnet 76)

Why is my verse so barren of new pride,
So far from variation or quick change?
Why with the time do I not glance aside
To 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 tell 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.