Read the following Japanese folktale.
Urashima Taro
Urashima Taro was a young fisherman in Japan. When he saw some children being mean to a small turtle, Urashima rescued the turtle and returned it to the sea.
The next day, a giant turtle swam to Urashima and spoke. "The small turtle you saved was Otohime, princess of the sea. She wants to thank you."
The giant turtle gave Urashima a ride under the sea, and Urashima stayed with Otohime for three days. Then he began to miss his mother, so he asked if he could go home.
Otohime was sorry to see Urashima go, but she said yes. She gave him a strange box called a tamatebako and said, "This box will keep you safe, but you must never open it." Urashima grabbed the box, climbed onto the giant turtle, and left.
Soon he was back home, but he found that everything had changed. His house was gone. His mother was gone. He asked if anyone knew Urashima Taro.
"He disappeared many years ago," the townspeople replied.
Urashima learned that while he had spent only three days under the sea, three hundred years had passed on land! He was so upset that he opened the tamatebako. White smoke poured out, and Urashima suddenly turned into an old man.
What is the main theme or lesson of the folktale?