himself with other people, wishing that he was more like the hopeful people he knows. He eyes other people enviously, wishing he had this man's friends and that man's artistic talent. All of this makes him feel discontented, even with the things he normally enjoys, and he ends up almost despising himself.

But then, he remembers his beloved, and his mood changes almost instantly. In line ten, he addresses his beloved directly as he says "Haply I think on thee." As he does so, his "state" alters to feeling suddenly uplifted, and he likens himself to lark singing at daybreak after a long night. He ends by saying that his beloved's love brings such "wealth" to him that he wouldn't trade places with a king.

In a nutshell, the narrator goes from dwelling on the negative—everything other people have that he wishes he had—to the positive as he remembers what he does have: his beloved, an incomparable treasure. He ends the poem with:

For thy sweet love remembered such wealth brings

That then I scorn to change my state with kings.

You could say that the speaker's mood changes twice. In the early portion of the poem, the speaker spirals into a low and dark mood. In the early lines, he becomes depressed due to failure and shame (or what he perceives to be failure). That's the first eight lines.

However, I suspect you're referring to the second change in mood, which starts on line 9. At that point, the change in mood is sudden and quite touching, actually. The narrator thinks of his beloved, and that is enough to change his mood. Just thinking of the person he loves, and the love that person holds for the speaker, is enough to completely transform his world. From the profound despair of the early lines, where the speaker is cursing his state and envying others, thinking about love is enough to raise his spirits just as profoundly, so much so that he no longer envies others.

That's a great poem!

In Shakespeare's Sonnet 29, the poet is despondent through the first two quatrains (groups of 4 lines).  Whether he is now "in disgrace with fortune and men's eyes" is not clear; the poet may simply be reflecting upon this condition.  At any rate, he sets up the condition as one which causes him a feeling of alienation and despair.  In this state, the poet declares that he is envious of the prosperity and companionship and talents of others in lines 5-8.

However, this despondency is broken in the third quatrain of this sonnet as he asserts, "Haply I think on thee,-and then my state...sings hymns at heaven's gate."  For, the love of one person can make all the difference to a person. In the ending couplet which sums up the meaning of the sonnet, the poet states,

For thy sweet love remember'd such wealth brings,/That then I scorn to change my state of being.

Having the this love, the poet considers himself rich and is content with his state in life.