Shakespeare's sonnets all share the same form, that is to say the same number of lines, the same rhyme scheme, etc. (Check out "Form and Meter" for the specifics.) What's more, he wrote 16 other sonnets about pretty much the same topic as this one, convincing this young man to have children. So in some ways the calling card is pretty clear. The poems look the same, they follow the same rules, and they deal with a lot of the same issues. What's fun, though, is seeing the variations that Shakespeare can come up with. He manages to make this theme fresh and interesting each time he tackles it. His images and metaphors are beautiful and fun to pull apart.