Who or what is the the subject of this poem?

In "Sonnet 29," Shakespeare is contrasting the world and all its troubles with the happiness and stability he obtains from his lover. In the speaker's everyday professional life, he is subjected to numerous setbacks and indignities. Life just seems so terribly hard and unfair at times. And when life starts to get the speaker down, as it so often does, he often wishes he were someone else, someone with much greater luck:

And look upon myself and curse my fate,
Wishing me like to one more rich in hope,
Featured like him, like him with friends possessed,
Desiring this man’s art and that man’s scope . . .

But then the speaker thinks of his beloved and how much joy their love brings him. Then he realizes just how much he has in life and no longer wants to be someone else:


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



Shakespeare is reminding us of what's really important in life. All too often we attach too great an importance to worldly fame, wealth, and reputation. Yet these are all things that are ultimately beyond our control, and so when they inevitably turn sour and we experience misfortune, our immediate reaction is to wish that we were someone else. But, as Shakespeare observes, if we were someone else, then we wouldn't be able to enjoy the love of the people who matter most in our lives.
https://www.poetryfoundation.org/poems/45090/sonnet-29-when-in-disgrace-with-fortune-and-mens-eyes