Why has fandom become so toxic?

It's getting difficult to like anything. Or rather, it's getting difficult to be a vocal fan of anything. In recent years, every aspect of entertainment has become as polarized as our political discourse. With warring encampments of fans waging flame wars on Reddit and Twitter, declaring yourself an admirer of something—usually science fiction—is tantamount to enlisting for active service.

I'm old enough to remember a time when meeting a person that shared an interest in comics or television was an enjoyable experience. Being a fan of something, while personally fulfilling, can often be a solitary endeavor. In the good old days, meeting a fellow fan in the wild alleviated that sense of alienation. Now, with the advent of the internet, it usually prompts aggression and annoyance. What changed to make fandom so toxic?

In the video linked above, Star Trek YouTuber Trekspertise provides his assessment of the issue. Correlating fractured fandoms with religions branching into various sects, Trekspertise attempts to humanize both sides of dueling legions of fans.