Quality is the main ingredient here, but mentality can help give a boost. Would not claim City are mentality monsters now that they've won no.Dont agree with that. It is much more about quality and way team is set up.
Real with their monster mentality have lost league to Barca by double digits and took a hammering from City in CL.
City who had not won CL and lost many ties in a way they shouldnt. They didnt all of a sudden become mentality monsters to win CL what happened was signing the right players and Pep playing in way that came through.
Folk just decide about mentality after the fact.
Playing wrong team and thinking likes of Busquets can keep up with top European teams is more important.. Busi could have the best 'mentality' in that side but still lack too much in his legs.
Madrid are without a doubt the side which have gone through when been the worse side most times and I'd also bet they're the side to turn the most games around in knockouts. It's part of the squad culture and it's being passed on from Modric, Kroos, Carvajal etc.
It's not a switch they can turn on whenever they need it and sometimes the opponent's quality is just too much, especially now with the old guard being washed. But it sure has helped them a lot in CL these recent years. The league is a totally different competition.