Shakespeare's Sonnet 29 is a love sonnet disguised as a poem about unfulfilled ambition and a career of disappointment and frustration. In lines 11-12, the speaker of the sonnet uses the simile "(Like to the lark at break of day arising/From sullen earth)" to compare himself to a happy bird at the start of a new day leaving behind the drudgery of the land and the limitations of gravity to enjoy all the freshness and opportunity a new day has to offer.

The speaker's happiness is deepened with a sense of gratitude and optimism, and all of these positive emotions come about as a direct consequence of the speaking thinking about his beloved. When the speaker is having a difficult time and questioning everything to do with his fate and fortune, all the speaker has to do is think of his beloved, and the worries evaporate:

For thy sweet love remembered such wealth brings
That then I scorn to change my state with kings.

The couplet that ends the sonnet employs a metaphor comparing the speaker with a king. That the speaker chooses to compare himself first to a lark and then to a king reflects two very different interpretations of the feeling of joy that the speaker experiences when thinking about his beloved, but both convey a sense of deep satisfaction and happiness.

In Sonnet 29, the speaker spends the first eight lines lamenting his "outcast state." He feels that other men have more skill ("art"), more friends, or more reason (hope) to feel good about themselves. But in line 9, he changes the tone and suggests, in line 10, that when he thinks upon his beloved, his state changes from the outcast, self-deprecating state to a heavenly state so wonderful that he would not trade it with kings. 

In lines 10-12, the speaker, as he thinks about his beloved, uses the simile comparing himself to a lark.

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; 

As he transitions from his self-loathing state ("From sullen earth,") to thinking of his beloved, he is moved to a much happier, higher plane of hope and love. The speaker compares this sense of rising to a higher place and the sense of awakening (from his melancholy) to the rising of the lark at dawn (rising of the day and of the sun). There is also the additional meaning of lark which means to do something for fun; this fits with his transition from a melancholy to a happy state.