At that time there were certain conventions in play-writing and poetry which lesser poets observed religiously. Shakespeare didn't. He used the forms and traditions while at the same time working incredible changes on them. Example: Romeo and Juliet is a tragedy, correct? Lots of killing and the "star cross'd lovers" tragically dead at the end. (I think they're a couple of hormonal idiots, but that's just me.) The change Shakespeare rings on the play is that it begins as a comedy (there are lovers separated by an obstacle--the families' feud--who nonetheless come together with the help of not one but two tricky servants (the Nurse, Friar Lawrence) and there is a wedding. Up until Romeo kills himself, the play could have been a comedy (I know, Tybalt and Mercutio die, but they're collateral damage of sorts. And hot-headed fools.). In fact, the Victorians hated sad endings and so re-wrote the last scene. Juliet wakes up in time; Romeo doesn't kill himself and they run away and live happily ever after. But the ending as originally written is, you know, tragic.
Same thing with the sonnets. The tradition form and themes are worked with and worked over. The sonnet tradition said, "Tell us what love is in your sonnet." William Shakespeare said, "Because I am an overwhelming genius, I will tell you what love is not in my sonnet." And he did:
Let me not to the marriage of true minds
Admit impediments. Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove:
O no! it is an ever-fixed mark
That looks on tempests and is never shaken;
It is the star to every wandering bark,
Whose worth's unknown, although his height be taken.
Love's not Time's fool, though rosy lips and cheeks
Within his bending sickle's compass come:
Love alters not with his brief hours and weeks,
But bears it out even to the edge of doom.
If this be error and upon me proved,
I never writ, nor no man ever loved.