At the beginning of the sonnet, the narrator laments about his life. He feels like an outcast and curses his fate. He also wishes that he was popular, wealthy, and more talented like other men. Suddenly, in the middle of lamenting, he thinks of someone special who makes him feel as happy as a lark singing hymns at heaven's gate. The speaker then recalls the person's "sweet love" and is filled with such spiritual wealth that he forgets about wanting to be someone else.
The gender of the person that the speaker adores has been debated for centuries. Regardless, Shakespeare is discussing the importance of having a companion. Whether the speaker is referring to a close friend or lover, it is clear that despite feeling lonely and isolated, all one has to do is think of the person that they love in order to feel happy. When reflecting on their "sweet love," the speaker no longer feels isolated and depressed. Essentially, happiness and a sense of spiritual fulfillment come from relationships with people one cares about.
Shakespeare's Sonnet 29 has as its meaning the fact that the love of another can make all the difference to a person. This fact is summed in the heroic couplet at the sonnet's end:
For thy sweet love remember'd such wealth brings,
That then I scorn to change my state with kings.
The love that the speaker feels is his bulwark against the isolation and despair with which he has long been familiar. In the first quatrain, for instance, the speaker says that he is in an "outcast state," cursing his fate envying the man who has friends. This despondancy, however, is broken in the third quatrain when the speaker "haply" thinks of his love, a thought that changes the darkness of his heart to the song of a lark.
The need for another is as old as man. Adam himself desired a companion, someone to love. For, "happiness was born a twin"; meaning and happiness depend upon one's sharing with a loved one. Otherwise, one feels isolated and empty.