Why do people fight over pairings in anime?

I would like to know why do some anime fans get all worked up over people liking certain pairings. Like for example, I have a friend on dA that is getting trolled to death just because she likes the Alucard/Integra pairing in Hellsing, they always try to say "that pairing's not canon!" or "if you like it, then you suck!" stuff like that and as for me I do Alucard/Tegra but also like other pairings too yet they don't have as much fighting like Alucard/Integra has. So my question is why do people fight always fight over anime pairings and why do they like to be mean to others just because they're a fan of a certain pairing. Please no flaming or rude comments. Thank you.