missing ("wanting"). The speaker is stating that he has done nothing to earn or deserve such a special love.
In the third quatrain, the speaker refers to the beginning of their relationship. He feels that when the young man first expressed any signs of love toward him, the young man did not know his "own worth" (line 9) or value at the time. Or, as the speaker suggests, the young man did not realize at that time how unworthy the speaker was for such a gift. Now that time has passed, the speaker believes and fears that the young man will change his mind about the speaker -- the young man will show "better judgment" (line 12).
In the final couplet the speaker still believes that he is unworthy of the young man's love and still thinks that the young man will leave him. However, he adds that his experience with the young man has been like a marvelous dream (another metaphor). He says that the experience was like dreaming that he was a king. However, upon waking, he finds and regrets that such is not the case ("no such matter"), that he is not really a king. Metaphorically, he is stating his feeling that his relationship will not and cannot continue.
This sonnet captures the feeling of uncertainty, a feeling that can and does occur in many relationships.