In Sonnet 6 Shakespeare uses the metaphor of loans and interest to suggest that the person he is writing to should bear children. "That's for thyself to breed another thee, Or ten times happier, be it ten for one; Ten times thyself were happier than thou art, If ten of thine ten times refigured thee." This is Shakespeare pleading with this man to have ten children, and that those ten children have ten children as well. There is also reference to Sonnet 5 where the metaphor of a flower is used. "Then let not winter's ragged hand deface In thee thy summer ere thou be distilled." Using the metaphor of a flowers essence being distilled so that in the winter when the flower dies you still have the perfume, Shakespeare tells his reader he should have children before he dies in the couplet of the sonnet. "Be not self-willed, for thou art much to fair To be death's conquest and make worms thine heir."
In Sonnet 13 Shakespeare sounds almost like he is yelling, or at least a little annoyed at his reader, again warning him about inevitable death and how he should prepare for this by procreating. "Against this coming end you should prepare, And your sweet semblance to some other give." He gets a little angry later on with, "Who lets so fair a house fall to decay,". The last line of the sonnet sums up the intention of Shakespeare's request quite well. "You had a father;let your son say so."
There are a few themes that seem to repeat at least in these two sonnets like beauty, a contrast between winter and summer, and death. It seems like all the themes written in these sonnets fit together, Shakespeare thinks this reader is beautiful and uses a winter/summer metaphor to kind of warn him about his inevitable death and hopefully make the readers beauty eternal by having children. It seems to me that Shakespeare was thinking a lot about death and how sad it is, "O that you were yourself!" that individuals beauty dies with them and the only way to continue it is through procreating.