I've seen it mentioned a few times recently that Shakespeare's couplets, both those at the ends of sonnets and those used to delineate the ends of scenes in the plays (where they were used, the most prevalent theory goes, to prepare the actors in the wings for their entrances), are generally considered to be of a lesser quality than the rest of his writing. If we assume this is so, for the sake of argument, one reason for it, in my opinion, is that Shakespeare seems to have been the type of writer who found it technically more inspiring to write toward something rather than to end something. This would also help to explain the similar decision, albeit on a much larger scale, to write the history plays in the order that he did, i.e. working backwards, essentially, via the use of what we now call prequels. But the best explanation that I've seen is found in W.H. Auden's Lectures on Shakespeare (published in 2000), in which he states, regarding sonnet 65:

"Notice how frequently the concluding couplets of the sonnets are poor. Unlike many of even the greatest artists, Shakespeare is not interested in completely flawless wholes. He says what he wants to say and lets the sonnet end anyhow. But that is the fault of the major artist, for a minor one always completes the work carefully. For instance, when we read Dostoevsky, we feel, yes, this is wonderful, this is marvelous, now go home and write it all over again. And yet if he did, the effect might well be lost. Most of us, however, can't get away with that attitude toward our writing." Here's the poem referred to:

Sonnet LXV

Since brass, nor stone, nor earth, nor boundless sea,

But sad mortality o'er-sways their power,

How with this rage shall beauty hold a plea,

Whose action is no stronger than a flower?

O, how shall summer's honey breath hold out

Against the wreckful siege of battering days,

When rocks impregnable are not so stout,

Nor gates of steel so strong, but Time decays?

O fearful meditation! where, alack,

Shall Time's best jewel from Time's chest lie hid?

Or what strong hand can hold his swift foot back?

Or who his spoil of beauty can forbid?

O, none, unless this miracle have might,

That in black ink my love may still shine bright.