Casillas had spent 11 trophy-laden years at Real Madrid before Jose Mourinho was appointed as the new Los Blancos boss in 2010.
The duo won the Copa Del Rey and the La Liga in their first two seasons together but it was during Mourinho's third year where their relationship turned sour.
After returning from a fractured hand, Casillas found himself on the sidelines for the final 13 games of the season.
Iker Casillas revealed how his relationship with Jose Mourinho fell apart at Real Madrid
And the 35-year-old has now revealed why communication with Mourinho broke down towards the end of his tenure at the Bernabeu.
“Initially we got along well but after a while things soured," Casillas told Spanish TV station Movistar+.
“We were like a couple who slowly see a relationship disintegrate.
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“In the beginning there was a lot of connection, but in the end there was not a good feeling.
“It's like a couple: at first it's very passionate, but then it does not work.”
Madrid face Barcelona this weekend in a Clasico that could well decide the destination of the La Liga title.
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Casillas is one of the most decorated players in Real Madrid's history
Casillas played in 37 matches against Madrid's bitter rivals and has backed his former side to be victorious in the race for the title.
"That rivalry exists and I have experienced it," he added. "In my experience, in a given game, we hurt Barcelona much more.
"My hope is that Madrid win and they can claim the league title because in my opinion, they have deserved it."