He also addressed Cleveland fans' negative reaction to his Miami move.
"It was easy to say, 'OK, I don't want to deal with these people ever again.' But then you think about the other side," he wrote. "What if I were a kid who looked up to an athlete, and that athlete made me want to do better in my own life, and then he left? How would I react?"
So he felt OK to return to Cleveland, where he feels he has unfinished business. In his first Cavs stint, he won two league MVP awards and went to the NBA finals once, but won no ring.
The interest was mutual, and the Cavaliers made their intentions known when they shipped off Jarrett Jack, Sergey Karasev, Tyler Zeller and a future first-round draft pick to clear the salary cap space necessary to sign James.
James joins a Cavaliers team that has struggled, making no playoffs and picking first three times in the NBA draft since James' departure. Those picks were two-time All-Star guard Kyrie Irving, UNLV's Anthony Bennett and University of Kansas phenom Andrew Wiggins, all currently in Cleveland's possession.
James: Miami was "almost like college"
In the SI piece, James said he'd still have left for Miami if he had to do it all over again. It could be described as the college experience that he never had.
"Miami, for me, has been almost like college for other kids," James, who went pro straight out of high school, said in Sports Illustrated. "These past four years helped raise me into who I am."
His ties to his home, he said, pulled him back.
"My relationship with Northeast Ohio is bigger than basketball. I didn't realize that four years ago. I do now."
Bringing trophy to Cleveland "most important"
James' Instagram account Friday had a photo, showing him in a Cavaliers jersey, said: "I'm coming home."
In the SI piece, James wrote that his "goal is still to win as many titles as possible, no question."
"But what's most important for me is bringing one trophy back to Northeast Ohio," he wrote.
"I'm not promising a championship. I know how hard that is to deliver," James added. "We're not ready right now. No way. Of course, I want to win next year, but I'm realistic. It will be a long process, much longer than it was in 2010."
White House takes notice
The news generated intense interest as soon as it broke, around 12:30 p.m. ET Friday. Three of the top 10 trending terms on Twitter were almost instantly related to James.
It even came up in the daily press briefing at the White House. When asked, White House Press Secretary Josh Earnest said he didn't know if noted NBA fan President Obama had a reaction to James' move.
But Earnest added: "The fact that he has made this decision is a testament to the kinds of values that he has incorporated into his life and he says that he is interested in instilling in his children."
"I think it is a pretty powerful statement about the value of a place that you consider home," Earnest said.