genuine question btw. i can't figure out the deal with capital f Fandom as a lifestyle. it's baffling to me. i'm a fan of things but identifying as being in the xyz fandom and performing acts of fandom is weird to me. it seems to get in the way of truly engaging with the art/media/whatever, but maybe i'm off base with this idk
KIND ANSWER:
people are social creatures, but it's hard to make friends unless your brain is shaped a certain way. many people find it nice to have a shared interest that gives you a conversational inroad with others, and even a sense of shared identity within a group. it can also be fun to compare and contrast in-depth analyses and opinions on the same piece of source material by others with a wide range of backgrounds. in essence, it is art transformed into community, often resulting in more art created as a byproduct. it allows people who wouldn't normally think of themselves as artists, critics or social people to engage with those aspects of themselves in a space that understands. it's neat!
PISSY ANSWER:
consuming media is an easy substitute for doing interesting or constructive things with your life. it's what capitalism uses to keep us from noticing we're all meat for the grinder, and in our modern times media companies try to foster an attitude of hyper-obsession over their properties to keep us guzzling from their content teats. toxic fandom culture is when people recognize something's wrong with the world, but they decide it's easier and/or more personally satisfying to strangle the piglet on the next titty over rather than kill the source of the milk
MY ACTUAL GUESS:
i think most people just kind of need to be a little totally ghoul-mode bugfuck about something in order to survive nowadays. fandom is one method of doing it. whether that manifests positively or negatively is Up To Them