The subject of the two sonnets is different. “Sonnet 116” is a love poem, and “Sonnet 29” is more self-centered. The tone of “Sonnet 116” is “I love you,” and the tone of “Sonnet 29” is “I am miserable, and you make me happy.”
The tone of “Sonnet 29” shifts from depression to elation.
The poem begins with sad remembrance and dejection, when the speaker is weeping. He bewails himself, and feels alone and dejected.
When, in disgrace with Fortune and men's eyes,
I all alone beweep my outcast state,
And trouble deaf heaven with my bootless cries
There has to be a dramatic shift for him to be so excited by the end of the poem. The poet seems to gather his courage, count his blessings, and decide to enjoy life.
Yet in these thoughts myself almost despising, Haply I think on thee, and then my state, Like to the lark at break of day arising From sullen earth, sings hymns at heaven's gate
“Sonnet 116” focuses on the speaker’s love. The speaker wants to explain how to love, or how he feels one should love.
Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove
Love is ceaseless and strong. It is unending across time. The speaker considers himself an expert, and in fact does explain what love is and what love is not.