Monreal told the club website: "It is very important, the atmosphere in the dressing room.
"It is mandatory to have a really good relationship with your team-mates, we are like a unit. You play like a unit and you spend a lot of time with them.
"I can see now there is only one group. We are together and in every moment the atmosphere is amazing - this is very important to get trophies. If you want to win something it is necessary to have a good group.
"In the last few years we have played with the same players. Every summer, we have signed one, two or a maximum of three players. The team is practically the same group so that is very important.
"Arsenal is not 11 players, we are 24 amazing players and when one is injured, the other player could play really well. We have really good players and really good people. In my opinion, it is better than ever."