Desiring this man's art (7)
Here, our speaker says he wishes he had some other guy's talent ("art"), which is why this line has led some readers to believe that the speaker of Sonnet 29 is a poet or playwright who has suffered from artistic rejection or failure and has had some kind of economic setback as a result. While we can't read this sonnet as an autobiographical record of Shakespeare's life, it's fair to say that Shakespeare knew a thing or two about being a "starving artist." Between 1592 and 1594, an outbreak of the plague closed down London theaters (Shakespeare's livelihood), which might have led him to start writing the sonnets. By the way, we know from Sonnet 18 that Shakespeare's speaker is in fact a poet, even though there's no real evidence of the speaker's profession in this particular sonnet.
For thy sweet love rememb'red such wealth brings,That then I scorn to change my state with kings. (13-14)
What's this? The speaker wouldn't trade places with even the wealthiest or most successful men on earth?! What's changed? For the first part of this sonnet, the speaker has been not-so-subtly suggesting that he's 1) penniless and 2) really unhappy about being broke. But, here, the speaker decides that the "sweet love" of a friend is enough to make him feel like he's rich, even if he doesn't have any money. In other words, you're never really "poor" if you've got someone to love.