Erza’s balancing act has always kept Natsu and Gray in check. Whenever the two of them would fight she would either use force or fear to stop the both of them. This time Erza uses her tears instead. Memories flood both Gray and Natsu’s mind, they had both decided that Erza shouldn’t have to cry. They realize their mistake and no longer continue their fighting; Fairy Tail is family and Erza reminds them of the masters words. Neither of them know that the master has given his life force to decimate as much of Alvarez as possible. Erza spares them that detail out of mercy or perhaps can’t bring herself to say what has happened, her own suffering is great.
This isn’t a common brawl between Natsu and Gray. They would fight over anything in the past. They have matured since their early infighting growing up in Fairy Tail. Its been some time since Gray and Natsu have had an all out brawl. This time each has a reason to actually destroy one another, and both have more than enough power to actually do it. Given the circumstances both are not thinking beyond themselves and their individual goals. One has to destroy Zeref, the other feels compelled to destroy END. Both have had someone taken from them, Lucy and Juvia.
Lucy and Juvia also play into the balancing act with Natsu and Gray. Juvia has provided support for Gray; Lucy has provided support to Natsu. The loss of Juvia leaves Gray bitter. The loss of Lucy leaves Natsu filled with hate. The new comers to Fairy Tail are now long integrated into the fabric of the guild. There is meaning in the loss of the two, not just an excuse to go on a rampage. Juvia has supported Gray for a long time, even if it was unwanted for a majority of it. He has taken Juvia more seriously and had promised to give her an answer concerning them. Lucy has been with Natsu for a long time. He had introduced her to Fairy Tail after all. Natsu feels an obligation to protect all the Fairy Tail members, though if there is more meaning to the relationship between Lucy and Natsu its not something that is spoken with words. It has helped lighten the load on Erza who has been balancing the two personalities for a long time.
Erza has also changed from the first time we see her in Fairy Tail. She has become a more well rounded personality. She seems less burdened by her past, having over come a great deal to get to where she stands today. In terms of the power gap between Natsu and Gray she no longer stands head and shoulders above. Her influence hasn’t diminished as she is able to end the infighting between them rather quickly. She risks physical harm to stop the both of them, as a token that she is willing to do anything to keep her family alive and intact. This version of the Fairy Tail A team is the most well rounded version we have seen of them yet and the most powerful they have ever been.