Sorbet's Kryptonite

Everyone knows the one and only weakness of Superman is the Kryptonite..
well, I have nothing to say about Superman but what I observed today seems quite similar.
I finally found a clue to Sorbet's unknown problem of being soft and quit flaring.
As I tried to open up the sheet between his tank and Maverick's, he starts to shrink his fins. So, afraid of a bad thing would happen, I put back the sheet. Then, I was anxious if the same thing would happen if I opened the other sheet which separates his tank with Hot Rod's. So, I tried and finally, Sorbet flared again! After a few days not flaring, he flared. But the strange thing is that he would shrink his fins whenever he sees Maverick but flares when he sees others. I tried this experiment of mine for several times and resulted the same thing. I guess, from now on, I'll put his tank beside other betta excluding Maverick's.