"I had so much fun during the first one, so I was like, 'You know what? Let's do it again," Alonso told reporters. "I just had a blast. It's a really special moment, and as long as I have an opportunity to do it, I want to do it. Because it's something I always dreamt about as a kid. ... It was a dream come true."
Guerrero, who is tied for the MLB lead with 23 homers this season, said he decided to forgo the event to "take care of myself physically and mentally."
"I feel good physically," Guerrero said through an interpreter. "Everything is going well. There is a long way to go still. I just want to keep it that way."
The 26-year-old Alonso, who has 11 home runs this year, won the Home Run Derby crown the last time it took place -- in 2019 at Progressive Field in Cleveland. Alonso edged Guerrero by a score of 23-22 in the last round to win the title.
The 2020 Home Run Derby was canceled because of the COVID-19 pandemic.
This year's Derby is scheduled for July 12 at Coors Field in Denver. Los Angeles Angels star Shohei Ohtani, who has 23 homers this season, will join Alonso at the tournament.