Inter Milan defeat in the Champions League final proves that winning is not only determined by the quality of the players, but also by having a winning mentality, which will boost the confidence of each player. We can see that in the match, after PSG scored a goal, the confidence of Inter Milan players wavered slightly, making it difficult to create a counterattacking momentum to even the score.
The quality of player is what many do think can get a team trophy but this proved to everyone big time that there is more to a team than just having set of good player alone, Inter Milan had their confidence that way because they probably were not expecting PSG to be a problem to them , if you had followed up with how PSG made it to the finals you will agree they struggled so hard at some point, even at the league stage they finished below top 10, so maybe Inter least expected much challenge from them.