How are Rikka's and Shichimiya's powers being affected? In the second season of "Love, Chunibyou and and other Delusions", Rikka's powers start weakening in the 9th episode. I don't get what's going on. Their powers are supposed to be imaginary. Then how is it possible for something imaginary to weaken unless the person imagines them to be weakening themselves. Rikka doesn't really have any reason to imagine that and apparently, from her reactions, she doesn't like it. Then what's causing it and why is it happening?

Furthermore, Shichimiya also tells Rikka about what once happened to her and that she decided to keep her powers and remain the magical devil girl. Does that mean that if she had fallen in love with Yuuta back then, her powers would have disappeared?

What do their powers have to do with Yuuta?
Rikka's imaginary "powers" are a reflection of how she perceives herself. The events of the series had begun to make her question whether it was time for her to "grow up" and give up on chuuni games; this internal doubt "manifested" itself as "weakened" powers.