"It was a happy accident," Wentz, 29, tells the December issue of Details. "But I think that certain things happen for a reason in your life, and maybe it was time to put the wild child in a cage."
Wentz says his wild-child days involved experimentation with prescription drugs, including one attempted overdose on Ativan (an anti-anxiety medicine) in 2005. But the drug-overdose death of Heath Ledger in January was an eye-opener for the Fallout Boy bassist.
"Dude, after Heath Ledger," he says, "I was just like, Man, this is not going to end up good."
Simpson-Wentz, 24, who married the rocker in an Alice in Wonderland-themed wedding in May, also played a role in his transformation.
"I realized I'd found my soul mate, and it made me want to be a better person," he says. "Now, with the baby, I want to be the best dad I can be."
"When me and Ashlee are on the red carpet together, I'm like her purse," he jokes.
With the birth of Bronx, a new Fall Out boy album dropping Dec. 16, and a clothing line, Clandestine, on his resume, Wentz vows he won’t be following in the footsteps of sister-in-law Jessica Simpson and her ex-husband, Nick Lachey, when it comes to reality TV.
"I get pitched, like, Newlyweds 2 once a week, dude," he says. "For like, move-to-an-island-after-this money. I just can't do it." Newlyweds 2 once a week, dude," he says. "For like, move-to-an-island-after-this money. I just can't do it."
Ashlee & Pete wed in Alice in Wonderland-themed ceremony
Read more about Hollywood's baby boom!