Because you've had bad experiences with women in the past. Or you've had no experience with women, but have heard bad things. Could be you've just been listening to betas/r9k/redpills/Hardcore MRA/whoevers as they tell you about how all women are Staceys or Trots or whatever catch word they've assigned as the current word for whore.
Or maby its none of the above.
But your at least aware of the cliche/stereotype/joke about women in relationships trying to change the man or forcing him to give up the things he enjoys or finds fun, because she disapproves.
While you may or may not truly believe this, it is something in the back of your head, niggling away. Perhaps that's why you've never had a girlfriend, because your worried she'll try and change you. Make you give up watching football with your friends and drinking beer. She'll definitely try and stop you from enjoying any sort of crud humor, like farting...
...But this girl, she farts like a guy! Then laughs at it! How could she possibly try and change your crud sense of humor, if she also has it? And if she won't try and change that, maby she won't try and stop you watching football and drinking beer with your friends! But not only that, she's cute too!
So you fall in love with a farting girl in a webm. Or more specifically you fall in love with a fantasy you created about a farting girl in a webm. And because your still young, feelings of love and affection manifest it's self as erections.