Ironically, at the poem's end, the solitude mentioned is in sharp contrast to the first line, for the poet now is in "the bliss of solitude."  It is what Emerson mentions in "Nature":

I am not solitary while I read and write, though nobody is with me.  [With nature I] am in the presence of the sublime."

This is the solitude in which the poet resides at the poem's end; it is the blissful harmony with the beauty and joy of Nature, not the melancholy of the first line. He is alone, but no longer solitary as he is in communion with Nature.

The poem starts off describing a cloud that is wandering all alone.  The words that Wordsworth picks to describe it emphasize his loneliness, alienation, and how he feels no connection with anyone around him.  He starts by saying, "I wandered lonely as a cloud," which is significant; note that he didn't say "as happy as a cloud" or "as beautiful as a cloud."  Also, he "wandered."  That indicates that he didn't have a destination or purpose--he was just wandering about, almost as if in search of a friend.  Then, he states that he "floats on high o'er vales and hills."  Note that he is far above the hills and vales, not connected to them whatsoever.  He is apart and separate, and not included.

However, as the poem progresses, Wordsworth slowly changes the lonely tone to one of inclusion and happiness.  The first clue is that he is pulled from his solitude to notice "a host of golden daffodils" that are beautifully dancing in a field below.  The sight is so cheerful, happy, optimistic and beautiful, that he is moved.  He states that "a poet could not but be gay, in such a jocund company," showing that the flowers have definitely made him very, very happy.  And, that happy sight stays with him long after he has seen it.  He states that later, when "on my couch I lie, in vacant or in pensive mood," he thinks of those flowers and his heart "with pleasure fills, and dances with the daffodils."  He has gone from a lonely person, wandering aimlessly, to someone who uses a beautiful sight to comfort himself when lonely.  When lonely, he imagines himself dancing amongst the daffodils, being with them, in their company, and happy.

I hope that those thoughts help; good luck.

William Wordsworth is considered one of the most important figures in British Romantic history.  British Romanticism placed an enormous emphasis on nature and humanity's relationship to it.  In this poem, Wordsworth feels isolated due to a separation from nature.  Floating lonely as a cloud symbolizes a separation from the natural world.  Through the course of the poem, he reestablishes that connection with nature as he moves through the field of daffodils and personifies the starry sky.  This poem shows that one is never truly alone, that they are always connected to nature, and in turn, a higher power.  In line 15 he says, "A poet could not but be gay," illuminating the importance of the poet's role in society during the Romanticism period.  Romantics such as Wordsworth and Emerson (American writer) believed it was the poet's responsibility to demonstrate humanity's connection to nature and relay the message to society.