"She talked about it all the time," Shelton said. "Like, 'Oh my God, I'm still working on it.'"
Then when she read them to him during the ceremony, Shelton told People he "couldn't believe how hard it hit me."
Stefani thoughtfully incorporated his father and brother, who have both passed, into her vows.
"The thing that got to me was she spoke about (them) as if she had known them almost," Shelton said. "She didn't make it seem that way, but it felt that way as she was speaking about them. She said she wished they were there and what kind of men she thinks they must've been. Man, it hit me. I felt like at this point, I've kind of gotten calloused. I guess I'm not because that absolutely slayed me. It's hard to even remember what else she even said. I'll never forget that feeling."
Shelton returned the wedding day surprise by writing a song for Stefani that he performed instead of delivering traditional vows. He started to co-write "We Can Reach the Stars" with Nashville songwriter Craig Wiseman on the way to work at "The Voice." The pair finished the song when he got to his dressing room, and as soon as he heard it, he knew he was going to release it – after their July wedding.
"I've got this lyric and this piece of music about my wife that I want people to hear," he said of "We Can Reach the Stars," which is out now. "I want to declare this about her to anybody that wants to hear it. Not releasing it didn't feel right because the song is as important as me wearing this ring. I want to shout it from the mountaintops. I'm married to Gwen Stefani, and I wrote a damn song about it. Listen, if you don't believe me."
The couple was married July 3 in an intimate ceremony in the chapel on their Oklahoma farm. Stefani wore a Vera Wang gown complete with a veil hand-embroidered with their names and the names of her sons, Kingston James, 15, Zuma Nesta Rock, 13, and Apollo Bowie, 7.
Shelton added a tuxedo jacket, vest and bow tie to his signature blue jeans for the big day. After the ceremony, officiated by Carson Daly, they dined on a multi-tiered white wedding cake, spent the night dancing and then closed their festivities with fireworks.