Los Blancos currently trail La Liga leaders Barcelona by 11 points and sit third in the table, five points behind local rivals Atletico Madrid.
And the shot-stopper says that it is reasonable for the tactician to become frustrated with poor results.
"When he gets angry with us, he is right because we didn't do well on the pitch," said the Spain international, who was answering question put to him from children at the city of Boadilla del Monte clinic.
"Since we are mature, we says things to face-to-face. Is he a nice guy? Yeah, he's a nice man. Putting aside that he is a coach, [as] a person, yeah, he's nice."
Casillas also said he is targeting a Champions League win this season and admitted that the side's La Liga challenge looked likely to end in failure.
“You don’t always crave what you have, so my immediate goal is to win Champions League,” said Casillas.
“For the fans and for me personally it is now a bigger [goal] to win the Champions League because I have not won it and we trail Barcelona by 11 points in La Liga.
“I would ask the Magi [the three wise men] that we cut the 11-point lead that Barcelona have and the five-point gap with Atletico Madrid, that would be a great gift.”
The 31-year-old also said that he would like to see out the remainder of his contract at the club he has served for over a decade.
“My idea would be to play to 35 or 36, which is when my contract is up with Real Madrid.
“Will I ever play at another club? Today I say no. This is my club and my home, but to do so in the future, it would have be in a team of outside Spain, a team that is not to in direct competition with Madrid.”