Miles has a more negative view of the spring than Jesse does.
The Tuck family drank from a spring that made them immortal. It meant that they never aged, and never died. It took some time for them to realize what had happened.
Of all of the Tucks, Miles probably lost the most. He had a family, but when the Tucks accidentally drank from the spring they lost the opportunity to be normal. Miles lost his family.
"I was more'n forty by then," said Miles sadly. "I was married. I had two children. But, from the look of me, I was still twenty-two. My wife, she finally made up her mind I'd sold my soul to the Devil. She left me. She went away and she took the children with her." (Ch. 7)
Miles lost everything because he was immortal. Clearly he still feels the pain from his wife's reaction and the fact that his children grew up without him.
Jesse sees the positives in the spring, but Miles does not. Jesse points out that although they only have each other, they are able to see a lot of things that other people don’t get a chance to see. When you can live forever, you never run out of time.
"We've never had anyone but us to talk about it to. Winnie—isn't it peculiar? And kind of wonderful? Just think of all the things we've seen in the world! All the things we're going to see!" (Ch. 8)
Miles reminds Jesse that not everything about the spring and living forever is a positive. He tells Winnie that there is more to it than “Jesse Tuck’s good times.” Living forever has meant losing his family for him, and Jesse never had the chance to have a family. Jesse still thinks about things from a child’s perspective, whereas Miles looks at things from a more adult point of view.