It's not about a girl being a dream or seeing something in a dream, it's about him writing his own dream (see line one).
What he's saying to this girl is that if he could choose what he dreams about it would be her.
If he could decide what his dreams would be about they wouldn't be about flying or driving fast cars or living in big houses, they would be about seeing and holding the woman he loves.
If the girl he loves is already "his," then the song means that even though he's around her all the time/lives with her/is married to her, even in his dreams he wants to be with her.
However, since he says that the end of the ending of the dream is "cruel" it probably means that the one he loves is either far away or doesn't love him back.
Either way he loves her very much.
It's not about a girl being a dream or seeing something in a dream, it's about him writing his own dream (see line one). What he's saying to this girl is that if he could choose what he dreams about it would be her. If he could decide what his dreams would be about they wouldn't be about flying or driving fast cars or living in big houses, they would be about seeing and holding the woman he loves. If the girl he loves is already "his," then the song means that even though he's around her all the time/lives with her/is married to her, even in his dreams he wants to be with her. However, since he says that the end of the ending of the dream is "cruel" it probably means that the one he loves is either far away or doesn't love him back. Either way he loves her very much.