The explanation given (per an Associated Press article - don't have the URL handy, so here's an excerpt):
It had rivet holes along the edge and the number 676EB stamped on it, convincing him he had found a piece of an aircraft. So he dragged the piece back to his family's vacation home.
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His parents dismissed it as a "piece of rubbish" that was probably debris from a boat, with his uncle making fun of him for dragging it around, but the 18-year-old insisted on bringing it back to South Africa to research the fragment.
"He was adamant he wanted to bring it home because it had a number on it," said Casper Lotter, adding that his son is not an aviation enthusiast but was simply drawn to the piece of debris.
"It just grabbed him for some weird reason," the father said in a telephone interview with The Associated Press.
Back home in Wartburg in KwaZulu-Natal province, the piece was stored with the family's angling gear and almost forgotten as Lotter focused on his final year in high school. His mother even tried to throw it out, he said.
It was only when Lotter read about another piece of possible debris from the missing airliner also found in Mozambique, about 186 miles (300 kilometers) from where he had made his discovery, that he resumed his probe.
"I was very shocked - Mozambique, similar color, similar area," the teen said of the piece discovered by an American man. "He described it similarly to what I'm looking at right now."
Last week, Lotter's mother Candace contacted Australian aviation authorities and they said the number on the part indicates it may belong to a Boeing 777, according to Casper Lotter.