rowboat is still. He uses this as ametaphor for his family's life: that everything is moving around them like a wheel, but they are stuck. Winnie must contemplate the idea of death. Everything is supposed to grow old and die at some point. Winnie is a young girl, and all these thoughts and emotions are very overwhelming for her. She spends the night with the Tuck family, thinking it all over.

In chapter 17, Winnie wakes up the next morning. This time, Miles takes her out on the rowboat. They go fishing in the pond. Winnie again must think about the circle of life and how death is a part of that. At first it troubles her. Even the thought of animals eating other animals is a bit upsetting:

"It'd be nice," she said, "if nothing ever had to die."
"Well, now, I don't know," said Miles. "If you think on it, you come to see there'd be so many creatures, including people, we'd all be squeezed in right up next to each other before long."
Winnie squinted at her fishing line and tried to picture a teeming world. "Mmm," she said, "yes, I guess you're right."

She is kind of glad when the fish gets away from her pole, but Miles reminds her that you can't put off death forever. After all these thoughts mulling in her mind, it is a mosquito that helps Winnie make her final decision:

A mosquito appeared and sat down on Winnie's knee. She slapped at it absently, thinking about what Miles had said. If all the mosquitoes lived forever—and if they kept on having babies!—it would be terrible. The
Tucks were right. It was best if no one knew about the spring, including the mosquitoes. She would keep the secret.

I would argue that the first factor which made Winnie appreciative of the need to keep Tuck's family secret is how nice the family had been to her throughout her ordeal. Even after kidnapping her, they do their best to make her comfortable and allay her fears.

Once Winnie is comfortable around the entire family, Tuck takes Winnie out on the pond in a rowboat that evening. He appeals to her sense of logic by explaining that every living thing is a part of life, and that growth and change are part of the circle of life. He explains that by contrast, Tuck's family has stagnated, creating a way of life that is both unnatural and frustrating. He helps her to understand that eternal life, without fear of dying, is not actually living at all.

The importance of no one finding out their secret lies in the fact that if everyone found out about the water, they would all drink it and thus be separated from life's natural progression. Tuck helps Winnie to understand that this would be a travesty and that people are far better off living a natural life that ends in death than living an unnatural one that never ends.