I agree with dstuva's answer here. But if you are being asked to tell what is negative about it, I will try to help.
The theme in general is not negative. It is about forgiveness. However, you can argue that it would be annoying if you got this poem addressed to you.
The reason for this, to me, is that the speaker is putting himself above the listener. He is saying that the listener is inferior. He's telling him "it's okay that you've screwed up, I forgive you." This is nice, but it implies that the speaker is better because he is giving the forgiveness. I think that it is somewhat arrogant to offer forgiveness unless it has been asked for.
The other thing you can point to is that the speaker talks about hating the listener (in addition to loving him). That is a bit negative as well.
Sonnet 35 by Shakespeare isn't generally negative, and isn't presenting a break up or anything like that. Reading it as if it were from a lover would be very comforting. The sonnet is reassuring and reconciling. The negative details mentioned are metaphorically compared to the mistakes a person can make or flaws a person can have. The idea is that no one is perfect and a person shouldn't be too hard on himself. The enotes Study Guide on the poem says the following:
In Sonnet 35, the friend is grief-stricken by his trespass, and the poet attempts to assuage his friend’s guilt through clever sophistry, reasoning that he himself must bear some of the responsibility for his friend’s offense. Because the friend’s transgression seems to involve sexual betrayal, a “sensual fault,” scholars have traditionally speculated that the friend may have seduced the poet’s mistress or been seduced by her, although a few commentators have concluded that the sexual suggestion is homoerotic.
If the wrong done the poet is moot, its effect is not. The friend is “grieved” by it. The poet argues, through parallel examples, that the faults of men have analogues in nature, even in the most beautiful of things: roses and buds, silver fountains, sun and moon. These images of nature’s perfection are no less subject to flaws than humans are. The rose stem bears wounding thorns, clouds and eclipses dim the beauty of moon and sun, canker worms devour sweet buds, and the water of the silver fountains may, at times, grow muddy. The poet then argues that he is also blameworthy, since he excuses his friend’s fault with his flawed comparisons and his faulty logic, which speciously justify his friend’s betrayal. He is thus corrupted by his need to excuse his friend’s faults.
The poem is written to a young man, the addressee of poems 1-126 of Shakespeare's sonnet sequence. The young man has done something to hurt the feelings of the speaker, and then felt devastated by what he did. The poem is an attempt to alleviate the discomfort of the offending young man.
The negative elements, again, are simply metaphors for flaws that humans possess.
Read it as if you are receiving this note from the love of your life. Does it sound like a love letter or like something negative?
Another strategy you can employ is to highlight the negative (ugly, sad, representative or legal or other opposition) words in the sonnet. This should give you a better idea of what is negative, and what the negativity represents to both parties in the sonnet.