Bellew challenges Ukraine's Usyk, who added the WBA and IBF titles to his WBC and WBO crowns in July by beating Murat Gassiev, on November 10 at Manchester Arena.
The 35-year-old Liverpudlian said at a press conference in Manchester on Monday: "Everyone asks why.
"I was on my honeymoon and for one reason or another I was watching Oleksandr beat up Murat Gassiev on a mobile phone.
"The fight stops, and then he makes the ultimate sin - he says my name. And it didn't take long. Soon as he said my name, I knew. I knew when I was watching the phone.
"The world's going to write me off and the more you write me off, the more I want to fight. So we will see. I'm going up against the monster, the man who no-one really wants to fight, the man who just dismantles fighters and beats them easily.
"There's only one difference when he looks in my eyes. He says he believes I am arrogant. But it is not arrogance he sees. For the first time in his whole career he sees a man looking back at him who believes he can win. I don't just think or believe - I know. I'm going to find a way. I always do."
Bellew, who is returning after two heavyweight victories over David Haye to the division in which he is a former WBC champion, also described Usyk as an "amazing champion" and a "superstar".
He added: "I have the one thing he doesn't have, and that's a punch that can switch lights off within the blink of an eye. Just one wrong step, one wrong manoeuvre, and it's over."
When asked if it could it be the last time for people to see him in a ring, regardless of the result, Bellew said: "If I don't stop after this she (his wife Rachael) is going to divorce me! We've only been married a few months. I don't fancy getting divorced, it's not part of the plan.
"She'll probably be watching this. Babe, I promise you this is - maybe - the last one!"
Usyk said: "I'll tell you why I chose Tony Bellew - because he is the man who takes the biggest challenges. He is not the kind of usual boxers who come, see how hard it is and just quit. He will come (and fight) to the end. These kind of fights are something the world wants to see."
Related links
