Shakespeare's Sonnets must be the most uneven work in the entire canon of English literature, slipping from blandness to a jarring intensity, and in Don Paterson's new book they receive a commentary to match. This isn't a conventional edition, though it includes Paterson's preferred version of the text and gives reasons for his choices. Its plan is more subjective: to write about poems that have receded into immortality as if their charge was fresh.

Don Paterson is a working poet (and a remarkable one), which gives a number of his observations unanswerable force. He resists the cryptographic approach exemplified by Helen Vendler, whose analyses of buried patterns he admits to finding seductive as well as exasperating, on the basis that such things aren't planned. Perhaps they can't even be avoided. They're the result of "instinctive decision-making, driven through the local compositional exigencies of the sonnet form".

Paterson's method is to avoid method. Since self-consciously critical readings kill the pace of the poems, he wrote the book "wide awake, bored, half-asleep, full of the cold, drunk or hungover", feeling "happy, frustrated, serene, elated, smart, befuddled and stupid". More questionably, he revised his notes as little as possible, in the hope "that this book has retained the feel of the 'reading diary' that it was".

His approach to the poems is broadly romantic, in the sense that "sincere" and "heartfelt" are his highest terms of praise. He has a preference for the kind of poem that "shows you the poet's excitement in not knowing what they're writing about exactly  only their commitment to use the weird procedure of poetic composition to lever out the truth from their own minds". But to think that your first thoughts are necessarily best is a cruder sort of romanticism. Artists know that spontaneity doesn't happen by itself. Don Paterson doubts that Shakespeare "ever revised a poem without improving it significantly", and he might have had more confidence about his own talents in that line.

In fact the tone of the book is rarely reminiscent of a diary, even if it's usually less hectoring than this oddly corporate outburst (loosely attached to the stylised self-denigration of Sonnet 72): "Thinking you suck is often the hallmark of a serious talent, and I am proud to say the poetry list I oversee [at Picador] is stuffed to the gills with men and women tormented by what they think of as their own wretched inadequacy, and their inequality to the demands of the art. This seems to have the effect of making them work harder than just about everyone else"

The Sonnets are a sort of theatre of voices, in which a Renaissance rhetorician, a neo-Platonist philosopher and an ardent, sometimes disgusted lover make themselves heard in different combinations. The voices in Paterson's book are more flatly contradictory: academic discourse, often lively, and internet chatroom rambling, in all its exhausted immediacy: "Just saying", "Argh! It's too much!" "feelin' the love", "We've all been there", "She treads on your dreams, dude", "Not much to see here, folks", "Group hug!", "Anyhoo I mean this as a diss to WS", "OK I'll get my coat", "Go figure", "done and dusted", "Is he on something?" "I love this guy", "Hey, it's a hobby", "So sue me", "does what it says on the tin". No reader of this book could guess at the fastidiousness of Paterson's poetry.

The voices often collide on the same page, as if "I love the admonitory and minatory tone of this poem, the way WS conceals his vulnerability, his great grief and disappointment, behind the dead baritone of this grim augur" wasn't profoundly at odds with the feeble joke eight lines later (riffing on a phrase of Vendler's): "I hope I never get invited to a nonce event. Har." This isn't just an incompatibility of tone but of substance. The experienced poet who feels that criticism should "honour the spirit of play in which the work was conceived" has nothing in common with the overwrought blogger who was saying, two paragraphs before, "The man's pain is intolerable." They communicate with us but not each other.

Part of Paterson's job, in presenting poems four centuries old, is to bring alive questions that were urgent in Shakespeare's day but have since lost their force. In such areas his blogger-self routinely sabotages any effort of reconstruction. Sonnet 68, for instance, denounces wigs, and there is an argument for dating its composition after the death of Queen Elizabeth, who might have taken offence. But how much respect are you showing the otherness of the past if you refer to Gloriana as "a noted syrup enthusiast" and to the wisdom of holding back poems "till Liz bought the farm"? You may feel that Shakespeare came as close to atheism as anyone in his age, but that doesn't justify referring to Christ as "our beardy imaginary friend".

It's refreshing to find a commentator who takes it for granted that Shakespeare's feelings towards the male addressee of the Sonnets were sexual ("Homosexuality is no longer considered either unnatural, deviant, outré, exotic, pathological or wicked, except by stupid and backward people, and is about as controversial a human attribute as left-handedness")  or it should be, except that it comes at the price of reconstituting the poet as a modern gay man in all but civil status. To imagine that Sonnet 92 refers "achingly" to marriage rights makes it seem as if Shakespeare and his young man were only waiting for a change in the bylaws to book their champagne reception on the London Eye.

The Dark Lady sonnets cause Don Paterson much distress with their "sexism", though sexism too is an idea with a cultural history. He describes some of them, long before their appearance in the sequence, as "toxic junk". By Sonnet 100 he is bridling at the adjective "resty" as applied to the muse, finding in it a hint of reflexive misogyny (the word was often used of horses, making it only a step up from "lazy cow" to his way of thinking  which would make sense if it was only used of mares). He acknowledges the power of Sonnet 129 ("Th'expense of spirit in a waste of shame") but tries to avoid the conclusion  "perhaps I can't"  that it's just a horribly misogynist poem. This seems oversensitive, considering that the poem doesn't actually mention women.

The no less electrifying Sonnet 144 ("Two loves I have, of comfort and despair") he dismisses as "a dog's dinner" with little interest for the contemporary reader. He might have preferred the sequence to have ended at 126. By 152 even the value of sincerity has worn thin for him: "I'm not saying that WS's feelings are not sincere; I am saying I don't believe they needed sharing." By this time he has shared with us his own feelings about the benighted misogyny of an imam in the papers, about the vow of celibacy in Catholicism turning healthy males into rapists of children, about the way poets can exhaust their readership by publishing a collection every three years. That's the real problem with Don Paterson's book. Not overinvestment in the poems, or even the poet, but the belief that prose can look after itself.