Not do much of him being cocky, but it's more of a dedication to his cousin who was in a car accident according to the Denver Post's Mark Kiszla.
“I fully believe in the Broncos winning the Super Bowl and am fully committed to it,” Miller said Tuesday, during a telephone interview. That was the whole feeling behind this guarantee. It wasn't because I felt all cocky," Miller continued. "This is bigger than me. This is all about my little cousin coming back from a serious accident."
Miller is dedicating the 2013 season to his 6 year old cousin, Jeremiah, who was involved in a car accident and just awoke from a coma.
Some might call it cocky or boasting but this guarantee is pure emotion. He' fired up for his little cousin and wants to win one for him.

Von Miller 