The 35-year-old Fall Out Boy bassist detailed the split in an interview with The Howard Stern Show on Tuesday. Wentz and Simpson divorced in 2011 after three years of marriage, and the musician said his youth and a period of depression contributed to the breakup.
"I thought it was forever," he shared. "My parents have been married, I think, 35 years. I don't really know. Forty years this year, maybe? But no, I didn't [see the divorce coming]. At the same time, we were both very young."
Wentz and Simpson welcomed son Bronx in November 2008, and the bassist and his band went on hiatus in 2010. The break caused the musician to lose a sense of purpose, and he found himself depressed and feeling unsupported by Simpson.
"I think there's an important thing where you know how to fight, because you can fight with somebody and it's not the end of everything," he related. "But if you don't know how to have those arguments, then they become nuclear. And we were doing this all in the public eye."
Simpson filed for divorce in February 2011, and currently shares custody of Bronx with Wentz. The bassist said he and the 30-year-old singer have figured out how to co-parent in "a really good, healthy way," and became friends after the split.
Wentz and girlfriend Meagan Camper welcomed their first child, son Saint Laszlo, in August 2014. Simpson went on to marry Evan Ross in August 2014, and is pregnant with their first child. The singer and her baby bump were spotted on a walk with Ross this week.