Famines and glut were part of the usual cycle of life in the Elizabethan world.
A poor harvest could mean starvation for many, as the storage facilities which we take for granted were unknown in those times. to thy sweet self too cruel - by refusing to procreate, hence denying a future to yourself.
See extended discussion of beguile = cheat; deprive of its due rights.
unbless = make unhappy, deprive of fruitfulness, and the pleasure of being married to you.
As the opening sonnet of the sequence, this one obviously has especial importance.
It appears to look both before and after, into the future and the past.
'Fair youth, be not churlish, be not self-centred, but go forth and fill the world with images of yourself, with heirs to replace you.
Because of your beauty you owe the world a recompense, which now you are devouring as if you were an enemy to yourself.
Take pity on the world, and do not, in utter selfish miserliness, allow yourself to become a perverted and self destructive object who eats up his own posterity'.
See also the further commentary on FRom faireſt creatures we deſire increaſe, That thereby beauties Roſe might neuer die, But as the riper ſhould by time deceaſe, His tender heire might beare his memory: But thou contracted to thine owne bright eyes, Feed'ſt thy lights flame with ſelfe ſubſtantiall fewell, Making a famine where aboundance lies, Thy ſelfe thy foe,to thy ſweet ſelfe too cruell: Thou that art now the worlds freſh ornament, And only herauld to the gaudy ſpring, Within thine owne bud burieſt thy content, And tender chorle makſt waſt in niggarding: Pitty the world,or elſe this glutton be, To eate the worlds due,by the graue and thee. A reference also to the increase of the harvest, by which one seed of corn becomes many.