What is Nick's view of the past, and what is Gatsby's opinion? Why is Gatsby's opinion unrealistic?

Nick believes the past cannot be repeated. Gatsby, however, is positive that it can be repeated. Gatsby's opinion is unrealistic because it doesn't allow for change.... it banks on feelings remaining exactly the same.

“I wouldn’t ask too much of her,” I ventured. “You can’t repeat the past.”

“Can’t repeat the past?” he cried incredulously. “Why of course you can!”

He looked around him wildly, as if the past were lurking here in the shadow of his house, just out of reach of his hand.

“I’m going to fix everything just the way it was before,” he said, nodding determinedly. “She’ll see.”

He talked a lot about the past, and I gathered that he wanted to recover something, some idea of himself perhaps, that had gone into loving Daisy.