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.