Replying to:
Reason #1
Let's start with why many join fandoms in the first place. Of course, they're drawn to the group, but above all, they want a sense of belonging, purpose, and identity. After a while, there may be inner conflict within the fandom. If they're heavily outnumbered in the conflict, they start to feel ostracized. This resentment causes them to become antis out of spite.
Reason #2
The parasocial relationship. K-pop fans become very emotionally attached to the group. When it seems like the group isn't reciprocating the love, some fans may start being disappointed and resentful. A common catalyst is the perception that the group is distancing itself from its core identity and humble beginnings. Some fans may feel that the group is selling out, neglecting day 1 fans in favor of larger audiences.
In conclusion...
Any reason for becoming an anti is irrational. It's okay to feel emotionally connected to a group or a fandom, but it's still important to recognize your individuality. You can still love and support the group even if you don't get along with some of your fandom. Excessive competition seems to be a common factor in inner-fandom conflict. In the end, you guys all love the same group, so whatever beef stemming from competition shouldn't matter too much. You'd all be going to the same concerts in person anyway. That's what matters in the end.
