Ah the imagination is such an interesting thing!
Dissapointment comes from things not working out the way we planned. We use our imaginations to plan for the future. Do other animals do this as well? Do dogs and cats daydream and plan for their week? Or just humans...i'm guessing probably just us.
Keats looks at this urn and thinks about how lucky the figures on it are, they will never get old, they will never change. Of course, these figures aren't real people, they don't have feelings, and they don't really love each other. Yet, Keats revels in the fact that even thought they will never have each other, their love for each other will never die. The girl will forever remain fair.
Someone once told me:
"We love someone the most the first day we meet them. Things are wonderful then. People fall out of love as they get to know each other. If you fell deeper in love, people would never break up."
I don't totally agree, but I do see what that person was trying to say. I have a friend who met this wonderful girl in college. They dated for three months, and then she was killed. He was devastated, and is left with only good memories of her. His next gf was jealous of the dead one, she said he loved her more than he'll ever love her, or anyone else.
In a way, she was right. Yet, the only reason the first gf is being held in such high regard is because they didn't have enough time together to start to annoy each other. They never had fights or big disagreements. Who knows what would have happened if she had lived, maybe they would have broke up and hated each other.
The point is...the things we never get...the melody we never hear...we always imagine as being that much sweeter.