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.