"The intensity, the smile, the infectious attitude, it carried over to all the other guys," said Humphries, who was shocked that Seau is now the eighth player from the '94 Super Bowl team to die.
Seau's mother appeared before reporters outside the former player's house, weeping uncontrollably.
Her son gave no indication of a problem when she spoke to him by phone earlier this week, she said.
Seau's death follows the suicide last year of former Chicago Bears player Dave Duerson, who also shot himself in the chest.
In October 2010, Seau survived a 100-foot plunge down a seaside cliff in his SUV, hours after he was arrested for investigation of domestic violence at the Oceanside home he shared with his girlfriend. The woman had told authorities that Seau assaulted her during an argument.
Seau's ex-wife told the Union-Tribune San Diego that he texted her and each of their three children separate messages: "I love you."