The theme of immortality runs strongly throughout the sonnets especially those adressed to the Fair Youth. However the theme of immortality may have been introduced because Shakespeare knew that Wriothesley was personally obsessed with immortality. We know this because at age 13 as a student of Cambridge University Wriothesley wrote an essay about fame in which he states " Everyman burns with a certain boundless lust for fame". Shakespeare would have known about Wriothesleys lust for fame and presumably his fear of mortal oblivion and was probably playing on them . Shakespeare forwards himself as the person who can offer The Fair youth both fame and immortality and this is likely why these themes feature so heavily in the sonnets.